Particle detector, system and method

ABSTRACT

The invention provides use of one or more emitted beams of radiation ( 16 ), for example, laser beam(s), in combination with an image capturing means ( 14 ), for example, one or more video cameras and/or optical elements to detect particles ( 30 ), for example smoke particles, located in an open space ( 12 ).

RELATED APPLICATIONS

This application claims priority to Australian Provisional PatentApplication No. 2004906488, filed 12 Nov. 2004 and entitled “ParticleDetector, System and Method” and, the specification thereof. Isincorporated herein by reference in its entirety and for all purposes.

FIELD OF INVENTION

The present invention relates to an improved sensor apparatus andimproved method of sensing. In particular the present invention relatesto an improved particle detector and method of detecting particles. Itwill be convenient to hereinafter describe the invention in relation tothe use of one or more emitted beams of radiation, for example, laserbeam(s), to detect particles located in an open space, however, itshould be appreciated that the present invention is not limited to thatuse, only.

BACKGROUND OF THE INVENTION

Throughout this specification the use of the word ‘inventor’ in singularform may be taken as reference to one (singular) or all (plural)inventors of the present invention. The inventor has identified thefollowing related art. There are a number of ways of detecting particlesin a region, such as a room, building, enclosure, or open space. Somemethods involve sampling air from the region and passing the sampled airthrough a detection chamber, whereby particles are detected and anestimation is made of the amount of smoke, for example, in the region ofinterest. Such an apparatus is exemplified in aspirated smoke detectorslike VESDA® LaserPLUS™ smoke detectors sold by the applicant.

Other detectors are placed in the region of interest, and use a sensorto detect particles adjacent the sensor. An example of such a detectoris a point detector, in which air passes between an emitter and asensor; and the particles are detected directly in the region ofinterest.

In both cases if the particles do not enter a sampling point (of theaspirated detector) or pass between the sensor and emitter of the pointdetector, no particles will be detected. As many buildings employ airhandling means for extracting air from a region, such asair-conditioning, there is no guarantee that suspended particles will bedetected rather than pass out of the region via the air handling ducts.It can be very difficult to use the aforementioned methods of detectingparticles in outdoor areas or very large indoor arenas where there maynot be appropriate locations to place a point detector or a sample pointand connecting tubing.

Other devices used to detect, for example, smoke include the detectordisclosed in U.S. Pat. No. 3,924,252, (Duston) which uses a laser and aphotodiode to detect light scattered from particles. This device uses acorner reflector to reflect the light back at the emitter. Dustonrequires a feedback circuit to detect whether the beam is emitted orblocked.

Another type of detector is known as a “Beam Detector”, which measuresthe attenuation of the intensity of a signal from a projected lightsource caused by smoke particles suspended in the projected light. Thesedetectors, namely beam detectors and the detector disclosed in Duston,have relatively low sensitivity and are only capable of measuring thetotal attenuation within the illuminated region.

The above noted detectors may need to address a number of difficultiesthat are faced when attempting to detect particles by use of emittedradiation in a monitored area that may comprise, for example, indoorrooms, large indoor arenas and outdoor areas. Some of these difficultiescomprise the following. The installation and commissioning of equipmentto provide emitted radiation and means for detecting the emittedradiation and/or scattered radiation may be onerous. In particular, suchequipment may be intrusive to the monitored environment and may requirecomplex connections, for example, wiring or otherwise to supply control,communications and power to the equipment. Additionally, a number oftechnical personnel with particular skills may be required to installand/or commission the equipment. Once installed and/or commissioned suchequipment may be susceptible to environmental conditions that form partof the monitored environment that contribute to drift, misalignment andthe like to cause inaccuracies of measurement. Furthermore, there areenvironmental conditions and events unrelated to alarm conditions thatmay commonly occur in the monitored environment and may contribute tofalse alarms when detecting particles. It is desirable to detectparticles in large rooms and areas and the physical distances that areinvolved may contribute to increasing the likelihood of the above notedenvironmental conditions and events having an effect on the efficiencyof detecting particles and also, the distances involved relate to thepath length to be traveled by radiation, which of itself requiresequipment with high sensitivity and error tolerance.

Nuisance particles such as airborne dust, for example, may be present inthe monitored environment and cause false alarms to be raised when thereis no actual threat of fire outbreak. For instance, smoke particles arethose generated as a result of thermal decomposition, such as in asmoldering fire, whereas nuisance particles may be generated without anunderlying fire threat by, for example, mechanical or biologicalprocesses. Light scattering characteristics are related to particle sizedistribution; and there are many types of smoke and many types ofnuisance particles and their particle size distributions often overlap.A light scattering method and apparatus using a light scattering cellfor chemically identifying individual particles of matter or multipleparticles of matter, such as found in aerosols, without collecting andchemically analysing the material is disclosed in U.S. Pat. No.3,901,602 (Gravatt Jr). According to Gravatt, in the case of singleparticle analysis, plane-polarized light is impinged on the particle andthe intensity of the light scattered into the plane of polarization overa specified angular range is measured. The intensity is related to theparticle's coefficient of absorption and its size. In multiple particleanalysis, the intensity of the light scattered into a planeperpendicular to the plane of polarization is also measured to determinethe total number of particles of matter. This information may be used tonormalize the intensity measurement of the first scattered light beam. Asmoke detector is presented by Gravatt as an apparatus embodying themultiple particle analysis technique whereby fire-produced aerosols maybe detected without interference from non-fire-produced aerosols ofsimilar density.

Any discussion of documents, devices, acts or knowledge in thisspecification is included to explain the context of the invention. Itshould not be taken as an admission that any of the material forms apart of the prior art base or the common general knowledge in therelevant art in Australia or elsewhere on or before the priority date ofthe disclosure and claims herein.

SUMMARY OF INVENTION

It is an object of the present invention to provide a method andapparatus for alleviating at least one drawback of the prior artarrangements.

In one aspect the present invention provides a method of detectingparticles comprising:

emitting a beam of radiation into a monitored region, and;

detecting a variation in images of the region with image capturing meanssuch that the variation in images indicates the presence of theparticles wherein the steps of emitting and detecting comprise:

determining an ON period of the beam of radiation and an exposure periodof the image capturing means in accordance with an indirectlyproportional relationship with a power level of the emitted beam.

In another aspect the present invention provides a method of detectingparticles comprising:

emitting a beam of radiation into a monitored region, and;

detecting a variation in images of the region with an image capturingmeans such that the variation in images indicates the presence of theparticles wherein the method further comprises the step of:

alleviating one or more of variations and the causes of variations inthe detected images that correspond to events other than the presence ofparticles of interest.

In a further aspect the present invention provides a method of detectingparticles comprising:

emitting a beam of radiation into a monitored region, and;

detecting a variation in images of the region with an image capturingmeans such that the variation in images indicates the presence of theparticles wherein the method further comprises the step of:

probing the emitted beam with a probe for commissioning the step ofdetecting.

In yet another aspect the present invention provides a method ofdetecting particles comprising:

emitting a beam of radiation into a monitored region, and;

detecting a variation in images of the region with an image capturingmeans such that the variation in images indicates the presence of theparticles wherein the method further comprises the step of:

dividing the beam into a plurality of segments;

determining a variation in images for each beam segment;

providing the determined variation in images for each segment to acontrol point so as to simulate a plurality of point particle detectors.

In yet a further aspect the present invention provides a method ofdetecting particles comprising:

emitting a beam of radiation into a monitored region, and;

detecting a variation in images of the region with an image capturingmeans such that the variation in images indicates the presence of theparticles wherein the method further comprises the step of:

determining the position of a predetermined geometric point in spacewithin the monitored region.

In still another aspect the present invention provides a method forsynchronising between a light source and an image capturing meanscomprising:

allowing the source to oscillate on and off at a pre-determined rate;

identifying the source in one or more video images captured by the imagecapturing means and;

continually modifying the image capturing means frame rate to remain insynchronisation.

In yet another aspect the present invention provides a method ofdetecting particles comprising:

emitting a first beam of radiation into a monitored region, and;

detecting a variation in images of the region with a first imagecapturing device such that the variation in images indicates thepresence of the particles and wherein the variation in imagescorresponds to backscattered radiation.

In still another aspect the present invention provides a method ofdetecting particles comprising:

emitting a first beam of radiation into a monitored region and;

detecting a variation in images of the region with an image capturingmeans such that the variation in images indicates the presence of theparticles wherein the method further comprises:

providing at least one additional beam adjacent the first beam fordetecting an imminent intrusion into the beam.

In still a further aspect the present invention provides a method ofdetecting particles comprising:

emitting a beam of radiation into a monitored region;

detecting a variation in images of the region indicating the presence ofthe particles wherein at least one of the beam of radiation and a meansof detecting the variation in images is adapted to communicate data.

In yet another aspect the present invention provides a method ofdetecting particles comprising:

emitting a beam of radiation into a monitored region;

detecting a variation in images of the region indicating the presence ofthe particles;

compensating for distortions in detected images.

In yet a further aspect the present invention provides a method ofdetecting particles comprising:

emitting a beam of radiation into a monitored region;

detecting a variation in images of the region indicating the presence ofthe particles;

applying a weighting function to detected images for selectivelyresolving image portions.

In still a further aspect the present invention provides a method ofdetecting particles comprising;

emitting a plurality of beams of radiation into a monitored region;

detecting a variation in, images of the region indicating the presenceof the particles;

adapting the beams to be sequenced in operation.

In still another aspect the present invention provides a method ofdetecting particles comprising;

emitting a beam of radiation into a monitored region;

detecting a variation in images of the region indicating the presence ofthe particles;

adapting at least one of a radiation source and a means for detectingthe images to be positioned in a controlled manner.

In yet a further aspect the present invention provides a method ofdetecting particles comprising:

emitting a beam of radiation into a monitored region;

detecting a variation in images of the region indicating the presence ofthe particles;

wherein the images are detected by image detectors boated in at leasttwo positions.

In still a further aspect the present invention provides a method ofdetecting particles comprising:

emitting a beam of radiation into a monitored region;

detecting a variation in images of the region indicating the presence ofthe particles;

supervising the beam of radiation.

In still a further aspect the present invention provides a method ofdetecting particles comprising:

emitting a beam of radiation into a monitored region;

detecting a variation in images of the region indicating the presence ofthe particles

masking a central portion of the detected beam so as to enhance thedetection of variations in the images.

In still a further aspect the present invention provides a method ofdetecting particles comprising:

emitting a beam of radiation into a monitored region;

detecting a variation in images of the region indicating the presence ofthe particles

checking the operation of an image capture means adapted for capturingthe images of the monitored region.

In yet another aspect the present invention provides a method ofdetecting particles comprising:

emitting a beam of radiation into a monitored region;

detecting a variation in images of the region indicating the presence ofthe particles

evaluating the detected images to compensate for interference with thedetected variation in images.

In other aspects the present invention provides apparatus adapted todetect particles, said apparatus comprising:

processor means adapted to operate in accordance with a predeterminedinstruction set,

said apparatus, in conjunction with said instruction set, being adaptedto perform one or more of the methods as disclosed herein.

Other aspects, preferred features and advantages of the presentinvention are disclosed in the specification and/or defined in theappended claims, forming a part of the description of the invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Further disclosure, improvements, advantages, features and aspects ofthe present invention may be better understood by those skilled in therelevant art by reference to the following description of preferredembodiments taken in conjunction with the accompanying drawings, whichare given by way of illustration only, and thus are not limiting to thescope of the present invention, and in which:

FIG. 1 shows a schematic representation of an embodiment of a detectorsystem from a side view;

FIG. 2 shows a top plan view of an embodiment of an image capture deviceand emitter position of the detector system of FIG. 1;

FIG. 3 shows a schematic perspective representation of an image taken byan image capture device of FIG. 2 in accordance with a preferredembodiment;

FIG. 4 shows a system overview workflow for signal processing for thedetector system of FIG. 1 in accordance with a preferred embodiment;

FIG. 5 shows a graphical representation of segmentation of data capturedby the image capture device in the embodiment of FIG. 1;

FIG. 6 shows a graphical representation of the integration of the datacaptured by the image capture device of the embodiment of FIG. 1;

FIG. 7 a-c shows images illustrating background cancellation performedby the detection system of FIG. 1 in accordance with a preferredembodiment;

FIG. 8 shows a graphical representation of a method used for calculatingpixel radius in an embodiment of the software used in conjunction withthe operation of the detector system of FIG. 1;

FIG. 9 is a top plan schematic view of a further embodiment of adetector system in accordance with the present invention;

FIG. 10 is a top plan schematic view of another embodiment of a detectorsystem in accordance with the present invention;

FIGS. 11 a-c are top plan schematic views of other embodiments of thedetector system in accordance with the present invention;

FIG. 12 shows a schematic representation of a part of the detectorsystem of FIG. 1;

FIG. 13 shows a schematic representation of captured image data from animage capture device of the detector system of FIG. 1;

FIG. 14 is a top plan view of another embodiment of the detector systemin accordance with the present invention;

FIG. 15 is a top plan view of a further embodiment of the detectorsystem in accordance with the present invention;

FIG. 16 is a top plan view of a further embodiment of the detectorsystem in accordance with the present invention;

FIG. 17 is a perspective view of a further embodiment of the detectorsystem in accordance with the present invention;

FIG. 18 is a top plan view of a further embodiment of the detectorsystem in accordance with the present invention;

FIG. 19 is a block system diagram of a further embodiment of thedetector system in accordance with the present invention;

FIG. 20 is an illustration of an optical arrangement in accordance withanother preferred embodiment of the present invention;

FIG. 21 is an illustration of an optical arrangement in accordance withanother preferred embodiment of the present invention;

FIG. 22 is an Illustration of an optical arrangement in accordance withanother preferred embodiment of the present invention;

FIG. 23 is an illustration of an optical arrangement in accordance withanother preferred embodiment of the present invention;

FIG. 24 is a top plan view of another embodiment of the presentinvention including a timing diagram indication signals in accordancewith the operation of a plurality of lasers;

FIG. 25 is a perspective view of another embodiment of the presentinvention;

FIG. 26 is a perspective view of another embodiment of the presentinvention;

FIG. 27 is a perspective view of another embodiment of the presentinvention;

FIG. 28 is an image view taken in accordance with the embodiment of thepresent invention shown in FIG. 27;

FIG. 29 is another image view taken in accordance with the embodiment ofthe present invention shown in FIG. 27;

FIG. 30 is a perspective side view of another embodiment of the presentinvention;

FIG. 31 is an image view taken in accordance with the embodiment of thepresent invention shown in FIG. 30;

FIG. 32 is another image view taken in accordance with the embodiment ofthe present invention shown in FIG. 30;

FIG. 33 is a perspective side view of another embodiment of the presentinvention;

FIG. 34 is an image view taken in accordance with the embodiment of thepresent invention shown in FIG. 33;

FIG. 36 is a perspective side view of another embodiment of the presentinvention;

FIG. 36 is a perspective side view of another embodiment of the presentinvention;

FIG. 37 is an image view taken in accordance with the embodiment of thepresent invention shown in FIG. 36;

FIG. 38 shows an optical element in accordance with a further embodimentof the present invention;

FIG. 39 shows an optical element in accordance with another embodimentof the present invention;

FIG. 40 is beam supervision arrangement in accordance with anotherembodiment of the present invention;

FIG. 41 is a perspective side view of yet a further embodiment of thepresent invention;

FIG. 42 is a perspective side view of still another embodiment of thepresent invention;

FIGS. 43 and 44 show image and beam profiles for beams used inaccordance with embodiments of the present invention;

FIG. 45 shows a masking structure in accordance with another embodimentof the present invention:

FIG. 46 shows a masking structure in accordance with another embodimentof the present invention and a beam profile in relation to the maskingstructure;

FIGS. 47 and 48 show illuminator means in accordance with respectiveembodiments of the present invention;

FIGS. 49 and 50 show perspective side views of respective furtherembodiments of the present invention;

FIGS. 51 and 52 show images taken in accordance with other embodimentsof the present invention;

FIGS. 53, 54 and 55 show images of regions of a beam used in accordancewith further embodiments of the present invention;

FIG. 56 is an image taken in accordance with another embodiment of thepresent invention;

FIGS. 57 and 58 show light source arrangements in accordance withrespective further embodiments of the present invention

DETAILED DESCRIPTION

In preferred embodiments of the invention, there is provided a methodand apparatus for detecting particles comprising emoting a beam ofradiation into a monitored region and detecting a variation in images ofthe region indicating the presence of the particles. More particularly,embodiments of the present invention provide an indication of thelocation of the particles. In essence, embodiments of the presentinvention provide a particle detection system, which provides foraddressability of detected particles, namely, their location by directdetection without the need for sampling the monitored environment orhaving to place a detector(s) in a useful location within theenvironment for particle detection. The beam of radiation may compriseone or more light beams emitted from one or more light source(s) and,variation of images of the monitored region or zone may be detected byone or more image capture devices such as cameras.

In further preferred embodiments of the present invention there isprovided a computer program product comprising:

a computer usable medium having computer readable program code andcomputer readable system code embodied on said medium for detectingparticles within a data processing system, said computer program productcomprising:

computer readable code within said computer usable medium for performingthe method steps as disclosed herein.

In a particular form the present invention provides a method ofdetecting particles comprising emitting a beam of radiation into amonitored region and detecting a variation in images of the regionindicating the presence of the particles wherein the method furthercomprises the step of modulating the beam of radiation. Further stepsembodying the method and features of preferred embodiments may includeidentifying an area of interest in the images, which represents acorresponding zone of the monitored region. Scattered radiation withinthe zone may be represented in one or more segments of a correspondingimage, which allows for the location of the particles in the region tobe identified. The location of the particles may be determined inaccordance with a geometric relationship between the locations of asource of emitted radiation, a direction of the emitted radiation and apoint of image detection wherein, the geometric relationship isdetermined from the images. The detected variation may be an increase inscattered radiation intensity. The increase in scattered radiationintensity may be assessed with reference to a threshold value. Theincrease in scattered radiation intensity may be calculated by averagingintegrated intensity values from the images. The method may compriseassigning different threshold values for different spatial positionswithin the region. The method may comprise directing the radiation alonga path and identifying a target in the images, the target representing aposition at which the radiation is incident on an objective surfacewithin the region. A location of the target in the images may bemonitored and the emission of radiation may be ceased in response to achange in the location of the target. The method may further compriseidentifying a location of an emitter in the images. Further, the methodmay comprise determining an operating condition of the emitter based onradiation intensity at the identified location of the emitter. Theimages may be processed as frames, which are divided into sections,which represent spatial positions within the monitored region. Also, themethod may comprise monitoring intensity levels in associated sectionsof the images and assigning different threshold values for differentspatial positions within the region, which correspond to be associatedsections.

In another embodied form, the present invention may provide apparatusfor monitoring a region, comprising:

an emitter for directing a beam of radiation comprising at least onepredetermined characteristic into the region;

an image capture device for obtaining at least one image of the region;and

a processor for analysing the at least one image to detect the presenceof, or variation of the at least one characteristic between the images,indicating presence of particles within the region.

The processor may be adapted to determine the location of particles inaccordance with a geometric relationship between the locations of theemitter, the directed beam of radiation and the image capture devicewherein, the geometric relationship is determined from the analysedimages. The apparatus may comprise a plurality of emitters, arranged todirect radiation along different respective beam paths. The apparatusmay further comprise one or more filters for adapting the image capturedevice to capture radiation from the emitter in preference to radiationfrom other sources. The filters may be one or more or a combination of:

a temporal filter.

a spatial filter.

a band-pass filter.

a polarising fitter.

The image capture device preferably comprises an attenuator. Theattenuator may comprise a variable aperture device. A plurality ofimage-capturing devices may be used. Preferably, the image capturedevice comprises a camera. It is also preferable that the emittercomprises a laser.

In a further preferred form, the present invention provides a method ofdetecting particles comprising the steps of: determining a path of abeam of radiation comprising placing a first image capturing device toview a source of the radiation and at least a part of the path of thebeam of radiation; communicating the position of the source to aprocessor; placing a second image capturing device to view an impactpoint of the beam of radiation; communicating related positioninformation of the impact point to the processor; determining the pathof the beam in accordance with a geometric relationship between theposition of the source and the position information of the impact point.

In yet another preferred form, the present invention provides a methodof detecting particles comprising the steps of: determining a region ofinterest containing a path of a beam of radiation comprising locating afirst point, being the position of a source of the beam, using an imagecapturing device; locating a second point being the intersection of thebeam of radiation with a field of view of the image capturing device,determining the path of the beam in accordance with the first and secondpoint; calculating a region of interest containing the determined beampath. The step of locating a second point may be performed with at leastone substantially or partially transparent probe and the probe ispreferably removed from the beam path once located.

In still another preferred form, the present invention provides a methodof determining the level of particles or, in particular, smoke particlesat one or more subregions in a region of interest comprising: directinga beam of radiation within the region, selecting a view of at least aportion of a path of the beam with an image capture device, determiningthe location of the source of the radiation relative to the imagecapture device, determining the direction of the beam relative to theimage capture device, dividing the beam of radiation into segments,determining a geometric relationship between the segments and the imagecapture device, adjusting a level of light received by the image capturedevice of each segment so as to allow for the geometric relationship.The segments may comprise at least one pixel and the segments arepreferably grouped to form the subregions for particle detection.

In a preferred form, the present invention provides apparatus adapted todetect particles, said apparatus comprising processor means adapted tooperate in accordance with a predetermined instruction set, saidapparatus, in conjunction with said instruction set, being adapted toperform the method steps as disclosed herein.

In embodiments of the present invention there is provided a computerprogram product comprising; a computer usable medium having computerreadable program code and computer readable system code embodied on saidmedium for detecting particles within a data processing system, saidcomputer program product comprising; computer readable code within saidcomputer usable medium for performing the method steps of the methods asdescribed herein.

In FIG. 1, an embodiment of a particle detector 10 is shown. Thedetector 10 is located in a region 12 to be monitored. The region couldbe a room, stadium, hallway, or other area. It is not necessary for theregion to be enclosed or indoor.

An image capture device 14 views at least a portion of the region 12,comprising a portion that contains electromagnetic radiation fromemitter 16. The image capture device 14 may be a camera or one or moredevices forming a directionally sensitive electromagnetic receiver suchas photodiodes or CCD's, for example. In the preferred embodiment, theimage capture device 14 is a camera. In the present embodiment, thecamera 14 uses full frame capture to capture the images to send analoguevideo information along communications link 18 to a processor 20. It isnot necessary to use full frame capture. However, it is preferable touse full frame capture for engineering simplicity in obtaining images,performance, and minimising installation restrictions. As would beunderstood by the person skilled in the art, other image capture devices14 such as fine transfer cameras may be used and methods to compensatefor the efficiency of full frame capture that is otherwise not availablein line transfer cameras, may be employed. Another communication link 22provides a connection between the emitter 16 and the processor 20. Theprocessor 20 controls the output of emitter 16, and/or receivesinformation about the output of emitter 16 through the communicationslink 22. Alternatively, the state of the emitter 16 may be sensed by thecamera 14 or determined automatically as disclosed below thus obviatingthe need for communications link 22. In the preferred embodiment, theemitter 16 is a laser producing visible, infra-red or other suitableradiation. The laser 16 may incorporate a lens 21 and spatial filtersuch as a field of view restrictor 23. When a beam of light travelsthrough a homogeneous medium there is no scattering, only whenirregularities are present does the beam scatter. Therefore, in thepresence of particles such as smoke particles the laser beam willscatter. Furthermore, in accordance with the preferred embodiment, thelaser 16 may be modulated, eg “laser on”, laser “off” in a givensequence. When no smoke is present, the intensity of pixels in acaptured image including the laser beam is the same regardless of thestate of the laser. When smoke is present, there is a difference betweenthe intensity of a captured image when the laser 16 is on (due toscattering), compared to the intensity when the laser 16 is turned off.

Optional filters are shown in FIG. 11 n the form of a polarizing filter24 and a band pass filter 26. The polarising filter 24 is adapted toallow electromagnetic radiation emitted from the emitter 16 to passthrough, while preventing some of the background light from entering thecamera 14. This is useful if the emitter 16 is a laser emittingpolarised light, then the polarising filter 24 can be aligned with thepolarisation angle of the laser beam to allow maximum transmission oflaser light, while removing some background light, which typically isfrom randomly or non polarised light sources. It is to be noted that thelight source does not need to be a laser for this to be achieved. Thesecond filter 26 is a band pass filter, which attempts to only allowlight within a predetermined frequency range (i.e. the frequency of theelectromagnetic radiation from the emitter 16). For example, aninterference filter or coloured gel may be used as the band pass filter26. By using a band pass filter (for example allowing substantially onlylight around 640 nm if a red laser of that frequency is used),significant background light will be removed, increasing the relativeintensity of light scattered from particles suspended in the air in theregion 12.

Other filtering methods comprise modulation of the laser and use ofpositional information with regard to the systems components asdescribed below.

The image capture device may employ an attenuator for controlling theradiation received. A controllable neutral density filter arrangementmay be used. Alternatively, the attenuator could be in the form ofcontrolling the intensity with a variable aperture. An optional,adjustable, iris 24 a may be used to control exposure levels. It can bemanually set at the time of installation, or the system couldautomatically set the exposure according to incident light levels. Thereason for this is to minimise or avoid camera saturation, at least inthe parts of the field of view that are used in subsequent processing.The iris 24 a may be a mechanical iris or an LCD iris or any other meansto reduce the amount of light entering the camera. Some electroniccameras incorporate an electronic shutter, and in this case the shuttertime may be used to control exposure instead of an iris 24 a. A spatialfilter 24 b is also shown, which may for example comprise a slit foreffectively masking the incident light to the camera 14. For example, aslit may mask the incident received light at the camera 14 to conformgenerally to the shape of the laser beam as it would be projected in theplane of the camera 14 lens. Items 26, 24 a, 24 b & 24 may be physicallylocated in a variety of orders or combinations.

In use, electromagnetic radiation, such as a red laser light fromemitter 16, passes through the region 12 and impacts on a wall or anabsorber 28. The field of view of the camera 14 comprises at least partof the path of the laser, and optionally, the impact point of the laseron a wall or other object in the region 12 that is a permanentstructure, which in this case impacts on an absorber 28. Particles inthe air in the region that intersect the laser, in this case representedby particle cloud 30, will cause laser light to scatter. Some of thelight scattered from particles will fall on the sensor of the camera 14,and be detected.

In the embodiment shown in FIG. 1 the camera 14 outputs analogueinformation to a video capture card 32 of the processor 20. The videocapture card 32 converts the analogue information to digitalinformation, which is then further, processed by computer 34. Theprocessing is undertaken by software 36 running on the computer 34. Inthe preferred embodiment, the processing is carried out in order tointerpret the captured image(s) such that an image plane corresponds toor is mapped to corresponding positions on the laser beam. This may beachieved by relatively straightforward geometry and trigonometry oncepredetermined location or position information of the system'scomponents is obtained.

In other embodiments it is possible to use a camera 14 which wouldcapture the data and transmit it digitally to the processor 20 withoutthe need for a video capture card 32. Further, the camera 14, filters24, 26, processor 20 and light source 16 may be integrated into a singleunit or units. Also, embedded systems maybe employed to provide thefunctions of at least the processor 20.

A number of camera 14 configurations may be used in this embodimentprovided image information in the form of data can be supplied to theprocessor 20.

In the example shown in FIG. 1, a laser modulator 38 is used to vary thepower of the emitter 16. The power level may be changed to suit lightingconditions, meet eye safety requirements and provide on/off modulation.In a preferred embodiment a high power laser to overcome ambientlighting may be used with short pulses to satisfy eye safetyrequirements. In particular, the effect of ambient lighting may bereduced by combining a higher power pulsed laser and a correspondinglyshortened shutter time on the camera. For example, assume that given alaser power of 1 mW and a normal laser pulse rate of 40 msec ON 40 msecOFF and that an F number of F5.6 is sufficient to give a requiredsensitivity indoors with a camera exposure time per frame of 40 msec.The difficulty is that bright sunlight of brightness N times the indoorbrightness requires the camera to be stopped down to avoid saturationwhich reduces sensitivity. In one form the invention provides anembodiment in which an approach is to reduce camera exposure time by afactor of N and reduce laser ON time by same factor of N whileincreasing laser power by same factor N. The laser may still be pulsedat the same frequency of say 12.5 Hz so the average laser power is thesame. The camera frame rate may also still be 25 frames per sec.Equally, the beam may be pulsed up to about 50 Hz and the frame rate maybe varied to about 100 frames per sec. The result is that the reducedexposure time of the camera allows the aperture to remain at the indoorsetting while bringing the intensity of sunlight ambient lighting backto the same level as indoor lighting. The higher power of the laserduring the reduced exposure time means that particle detectionsensitivity stays the same as indoors. With respect to eye safetystandards, the question may still remain whether a higher power pulsedlaser is acceptable. In answer to this, one preferred aspect of theinvention provides that the primary light beam may beneficially bepulsed ON, in synchronisation with the camera shutter-open period, for aduration shorter than normal camera frame duration. This gives thebenefit that a higher output light power level may be used, and anincreased camera aperture, whilst still avoiding saturation of thecamera by high ambient lighting. This allows the system to functionsatisfactorily in high ambient lighting conditions, whilst alsoremaining conformant with eye safety standards proscribed in variousregions of the world. These eye safety standards define the laser powerthat may be used in a populated open area in a manner that allows thepeak laser power to be increased at reduced duty cycles. For example,industry standards permit a Class 2 visible laser operating at 12.5 Hz(half the standard 25 Hz camera frame rate) with an ON period of 40 ms,to have a peak output power of 1.18 mW. In one embodiment the same laseris operated at a reduced ON period of 0.1 ms and may then operate at5.26 mW. Under these circumstances the sensitivity of the system may bemaintained with a more than four-fold tolerance to increased ambientlighting. Likewise it is envisioned that the ON period may be extendedto 100 ms or in fact to a duration of about a few seconds for much lowerpeak output power and alternatively the peak output power may extend upto 500 mW with a correspondingly shorter duration of the ON period. Inan alternate form, the ON period of the beam may be greater than orequal to the exposure period of the camera.

The camera 14 shown in FIG. 1 may capture 30 frames every second, theemitter 16 is cycled on for one frame and off for the next. The amountof light in a region is sensed for each frame, and the sum of the lightin a region when the laser is off is subtracted from the sum of lightreceived while the laser is on. The sums may be over several frames. Thedifference between the sum of light received when the laser is oncompared to the light received when the laser is off is taken as ameasure of the amount of scattering in that region. To act as an alarm,a threshold difference is set and should the difference be exceeded, thealarm may be activated. In this way the detector 10 may act as aparticle detector. As measuring the scattered light from particles isknown to be a method of determining whether there is smoke in a region,the detector 10 may be used as a smoke detector.

The detector 10 may be set to wait until the measured scattering exceedsa given threshold for a predetermined period of time, before indicatingan alarm or pre-alarm condition. The manner for determining an alarm orpre-alarm condition for the detector 10 may be similar to the methodsused in aspirated smoke detectors using a laser in a chamber, such asthe VESDA™ LaserPLUST™ smoke detector sold by Vision Fire and SecurityPty Ltd.

FIG. 2 shows a top view of the embodiment in FIG. 1. The camera 14 has afield of view θ, which in this case covers substantially all the region12, which may be a room in a building. The light from emitter 16 isdirected generally towards the camera 14, but not directly at the lens.There is therefore an angle subtended by an imaginary line between thecamera 14 and the emitter 16, and the direction of the laser beam. Theangle may be in the horizontal plane as shown by angle z in FIG. 2,and/or the vertical plane as shown by angle x in FIG. 1. The laser beamdoes not impact on the camera lens directly. Nonetheless, the laser beampath will be in the field of view of the camera 14, as shown in FIG. 3.

Physical System Variations

It is desirable in some circumstances to use a number of emitters in asystem. This may be to comply with regulations, provide back up, or toassist in covering a larger area than could be covered with a singleemitter.

If coverage of a large area is required, it is possible to employ anumber of emitters so that smoke may be detected in a number ofdifferent locations within a region. FIG. 9 shows an arrangement wherebycamera 50 is located within a region such a room 52. If detection wasrequired across a large area, multiple lasers 54 and 55 could be spreadaround the room to provide coverage. FIG. 9 shows the emitters groupedinto t groups, with emitters from group 54 targeted at point 56 andemitters 55 targeted at point 57. The camera 50 may have the points 56and 57 in view, or may not see the points 56 and 57. Camera 50 may havepoints 56 and 57 in view by way of an optical arrangement to project animage of points 56 and 57 into the field of view of camera 50, forexample, rear view mirrors (not shown in FIG. 9) placed forward ofcamera 50. Likewise a prism or some other optical system could achievethis result. Further, the emitters 54 and 55 may all be onsimultaneously, or may be cycled, so that if the camera 50 can detectthe point at which the radiation lands, the radiation detected in thecamera can be used to verify that the emitter is operating and notblocked. Detection of individual emitters is possible if they wereswitched on and off sequentially, or in any sequence of patterns thatare not linearly dependant, so that using timing information, it ispossible to detect which emitter is on at any one time. Further, knowingwhich emitter was firing would allow the detector to localise subregions in the area to be protected and ascertain where any detectedparticles were located with respect to the sub regions. In effect thebeam or beams that have been scattered by particles may be determined.

The emitters 54 and 55 do not all need to intersect on targets 56 and57, and may be distributed along a number of targets, or cross over eachother onto other targets.

An alternative is shown in FIG. 10, where the lasers 58 and 59 are aimedaway from the camera 60. The camera 60 can detect a light from the laserlight hitting the wall at point 61 and 62. If either of these pointsdisappears, then the detector system knows that either a laser is faultyor something is blocking the path of the laser light. If the laser isblocked, generally the object blocking the laser light will also reflectthe light, and therefore the laser spot will shift from the known targetarea, that is original point 61 or 62. The camera can detect the shiftin the spot and may sound an alarm or turn the laser off. This may beimportant, especially if the laser is not considered eye safe. Anothermeans by which faults may be detected is when a spurious object such asa spider web intersects with a beam causing scattering of the emittedradiation. The silk thread commonly left dangling by spiders when theydescend from ceiling to floor level is an example of nuisance objectsthat although often nearly invisible to the human eye under normallighting conditions, may be readily detected by the system of thepresent invention and can easily generate a signal equivalent to aparticle density that requires an alarm response. Other nuisancematerial that may remain suspended in the beam may comprise the nylonline often used to suspend signs and warning notices from ceilings inapplications such as retail or decorations such as Christmasdecorations. If the sign or decoration itself were suspended at theheight of the beam this would necessarily cause an alarm or a fault tobe identified and reported, but it is undesirable to report an alarmmerely because of the supporting thread.

Any signal from scattering off an object such as a spider's web or otherlike material may suffer sharper spatial transitions than particles ofinterest such as smoke it is also noted that fine objects such as aspider's web are sensitive to polarization rotation. While in operation,it is possible that small amounts of solid material will enter the laserbeam and remain effectively fixed, causing a significant amount of lightscattering that could be falsely identified as being due to smoke and socause a false alarm. Several methods may be used to address thisproblem:

In one embodiment, the laser beam diameter may be made wide in orderthat the thin fibre intersects only a small fraction of the beamcross-sectional area, and so produces only a small signal, below thealarm threshold. If this small signal remains constant over time (e.g.with a time-constant of 2 hours or more), then it may be subtracted fromthe reading obtained from that location so as to maintain long-termcalibration accuracy.

In another embodiment, occasional movement of the emitted beam, forexample by translating the emitter in a lateral direction, may obviatesuch false detections of scattered radiation. The emitted beam or beamsmay be translated in directions perpendicular to the beams' direction ofpropagation. In particular, the laser beam(s) may be momentarily pannedso as to give a lateral displacement at the location of the nuisancesignal of, say, 50 mm. If the scattering is being caused by smoke thenthe signal will vary very little as the beam is moved. If a danglingthread, or the like causes the signal, then it will vary sharply.

In FIG. 10 a second camera 63 is shown which may be connected to thesystem to provide additional views. Using two cameras may allow a moreaccurate means of locating the area of smoke than using a single camera.Also, the additional view will provide scattering information fordifferent scattering angles for the same particulate material. This datacan be used to discriminate between materials with different particlesize distributions or scattering properties, This in turn can be used toreduce the system sensitivity to nuisance particles that might otherwisecause false alarms such as dust, for example. With the use of one ormore emitters, variation in scattering angle; wavelength of emittedradiation; polarisation rotation; plane of polarisation of viewedscattering and varying the timing of emission and detection all providemeans for discriminating between different types of particles.

Given that large particles (often associated with dust) forward scattermore than smaller particles (commonly caused by fire), a determinationof particle type can be made. If there is significantly more forwardscatter than side scatter for a particular segment of the emittedradiation path, then it may be interpreted that the particle density atthat particular segment consists of a proportion of large particles. Itmay be useful to compare this to other segments or other times, in orderto ascertain characteristics of the event that caused the particles tobe present in the first place. In a particular embodiment, dustrejection may be achieved by exploiting scattering angle. In this aspecttwo cameras per laser beam may be used, one at a very shallow angle (say1 degree), the other at a larger angle (say 30 degrees). The firstcamera will have much greater sensitivity to large particles (dust). Aproportion of its reading may be subtracted from the other camera toreduce sensitivity to dust. The incidence of false alarms may beusefully reduced if the characteristics of the light scattered from theairborne particles is analysed and compared to the known scatteringcharacteristics for a range of smoke types and nuisance particles. Thepresent invention provides a method of determining these characteristicscomprising measurement of the scattered light signal strength at varyingangles, planes of polarisation and wavelength.

In FIG. 11 a camera 64 views two lasers 65 and 66 that cross the room.FIG. 11 b uses a laser that is reflected back towards the camera 67, toprovide better room coverage and capture both forward and backwardscattered light.

In the present embodiment, the processor 10 comprises a personalcomputer running a Pentium 4 chip, Windows 2000 operating system.

An aspect of the present embodiments is signal processing discussed indetail below with reference to FIG. 4 which is a data flow diagram, thelayout of which, would be understood by the person skilled in the art.For ease of reference, the signal processing in this embodiment isconducted using software for the detector 10, referred to generally asthe software. It is to be noted with reference to FIG. 4 that the dataflow lines indicate image data flow (2-dimensional array data), arraydata flow (1-dimensional array data) and simple numeric or structureddata flow at different stages of the processing. Thus, some of theprocessing functions described may handle the more intensive image dataor optionally, the less intensive numeric data, for example. As would beunderstood by the person skilled in the art, engineering efficienciesmay be attained by choice of the components and software entities usedto carry out the processing functions at these respective stages.

Laser State Determination

At step 401 of FIG. 4 a determination of the laser state is performed.The software in this embodiment relies on having the laser source withinthe field of view of the camera in order to determine the state of thelaser for a particular frame.

A small region of interest is assigned that includes the laser sourceradiation. The centre of the region is set to an initial position of thelaser source spot. The average pixel value in the region is computed. Itis then compared with a threshold value to make the decision of whetherthe image records the laser on or off.

The threshold value is the average of the outputs of a peak detector anda trough detector that are fed by the average. Each detector executes anexponential decay back to the current average in the case that a newpeak or trough has not been made. The time constant is set in terms offrames, preferably with values of about 10.

This technique has proven to be fairly robust. An alternative method isto look for one or more pixels that exceeded the average in therectangle by a fixed threshold.

In an implementation where the laser on/off switching is more closelycoupled to frame acquisition this function may not be required. However,it can still serve a double check that the laser source is not obscuredand is of the correct intensity.

Laser Position

At step 401 of FIG. 4, a centre of gravity algorithm estimates the pixelco-ordinates of the laser source within the area being monitored. Thispositional information is optionally updated at every “laser on” imageto allow for drift in either the laser source or camera location due tomovement of the mounts and/or building over time. The factors affectingthe stability comprise movement of walls within the building, mountingpoint rigidity etc.

More precisely, the threshold established in the previous step (laserstate determination) is subtracted from the image and negatives, areclipped to zero. The centre of gravity of the same rectangle used in thestate determination then yields (x,y) co-ordinates of the laser spot inthis calculation, the pixel values are treated as weight.

An alternative technique is to treat the previously described area as animage and calculate an average of a large number (˜50) of known “emitteroff state” images, then subtract the average from the latest image thatis known to have been captured with the emitter on. The previouslydescribed centre of gravity algorithm is then applied to the image datato estimate the position of the spot.

Compute Regions of Interest & Background Cancellation

At step 403 of FIG. 4, regions of interest are calculated. At step 404of FIG. 4, background cancellation is performed. A combination ofinterpolation and frame subtraction is used during backgroundcancellation to reduce interfering temporally variant and invariantinformation from the image. The image is segmented into three regions ofinterest as shown in FIG. 5. The background is segmented into backgroundregions 101 and 103, and there is an integration region 102. Theseregions are updated periodically to reflect any detected changes in thelaser source location. The choice of shape of the regions of interestreflects the uncertainty in the precise position in the image of thescattered radiation. In FIG. 5 the camera cannot see the point where theemitted radiation hits the wall (which occurs beyond the left hand edgeof FIG. 5), and therefore the exact path of the emitted radiation isunknown. This produces a region of interest 102 that expands as thedistance from the emitter increases. A method of determining the path ofthe emitted radiation manually is to test the location of the emittedradiation by blocking the radiation temporarily and checking itsposition, then entering the data manually into the processor.Alternatively, one or more substantially transparent probes, which maybe in the form of articles such as plates, may be inserted into thebeam. Some scattering will occur on entry and exit from the plateproviding a reference point or points in the image from which therequired integration area and background areas may be computed. Inapplications where the detector may be used for detecting particles in,for example, clean room or hazardous environments, the windows of suchenclosures may act as the substantially transparent plates and, thesetherefore may establish the path of the beam without the need to intrudeinto the environments to install the detector system components.

In general a probe or probes that are useful in commissioning thedetector use a light-scattering translucent body to indicate to thesystem the path of the laser beam at one or more points along the beam.As noted, this is to verify that the beam passes where it is intendedand that the locations along the beam are being correctly mapped. It isalso useful to demonstrate the correct response of the system, withoutneeding to generate smoke in the area, which is often highlyundesirable. In applications where the position of the beam may beaccessed from ground level using a pole (which may be telescopic ormulti-part) it is convenient to attach a sheet of (preferably stiff)translucent material to such a pole. For example, for the purposes ofsimply intercepting the beam and confirming that the system identifiesthe correct location of the interception a piece of plain white paper,of for example A4 or letter size, supported on a wire frame may beadequate. In a preferred embodiment, a more sophisticated and usefulapproach is to use a piece of material with light scatteringcharacteristics that approximately match that of smoke at a knowndensity. For example, a thin sheet of glass loaded with small particlesof aluminium oxide may be used to scatter approximately 1% of theincident light, which also permits measurement of the effectivesensitivity of the detector at that point, and by inference, at allother points in the beam. A three dimensional object rather than a flatsheet may also be used, and may be preferred in some circumstances sincemaintaining orientation is not then a problem. An example would be aglass bulb, or an inflated balloon of a suitable wall colour andthickness. The latter may even be helium filled and introduced into thebeam on a tether from below. Where the laser beam passes through a spacethat cannot be readily accessed from ground level (for example a sportsstadium, or a building atria, some of which are 50 metres and more aboveground level) other methods to place the scattering medium into the beammay be required. For example, a small radio-controlled flying device maybe used, preferably a rechargeable electric helicopter suitable forindoor use. It is not necessary for this device to be held stationary inthe beam for a significant period of time (eg>50 msecs), but merely tocross it on at least one occasion while the laser is on. A suitableexample helicopter is the Sky Hawk R/C Mini Helicopter model HP4034,manufactured by Toy Yard Industrial Corporation of Shantou City, China.

The purpose of a narrow integration area is to reduce the noisecontributions from pixels that are not contributing a scattering signaland also to allow the background regions to be closer to the integrationregion thus allowing a better estimate of the correction factor that isused for correcting the illumination level in the laser off images.

The integration region 102 contains the emitted radiation path, whilethe areas to each side, background region 101 and 103, are used duringbackground cancellation. The regions are generally triangular, that iswider further away from the laser source. This is necessary becausewhile the exact location of the radiation spot is known, the exact angleof the path is not, so a greater tolerance is needed at the other end ofthe path when the camera cannot see where the radiation terminates.There is more noise in a fatter section of integration region due tomore pixels, fortunately, each pixel represents a shorter length of thepath, so the larger number of samples per unit length allows moreaveraging. If the camera can see the radiation termination point, therewould be less uncertainty of its position and the regions of interestwould not need to diverge as much as shown in FIG. 5.

Two background regions 101, 103 are chosen for interpolation of thebrightness compensation factor for correcting temporal variations inbackground lighting on either side of the radiation path in the laseroff images. For example, changes in lighting due to two different,independent temporally varying light sources on either side of theradiation path. This principle could be further extended to allow forvariations along the path, not just to either side of the path bysubdividing the three areas 101, 102, 103 into segments along the lengthof the radiation path and performing the calculations for eachsubdivision.

The background cancelling algorithm sums n “on frames” and m “offframes”—the sequence of these frames is arbitrary. Prior to thesubtraction of the “emitter off” frames from the “emitter on” frames,the Remitter of frames are scaled by a factor, f, to compensate forvariance in rumination levels of the images. This may be useful withartificial lighting, the intensity of which varies rapidly. Theresultant image contains any differences between the n “emitter on” andm ‘emitter off’ images. This is shown graphically in FIG. 6.

The scaling factor f is determined by interpolation, using the ratios ofbackground variation between the laser on and laser off frames.

$f = \frac{( {\frac{\mu_{{on}\; 1}}{\mu_{{off}\; 1}} + \frac{\mu_{{on}\; 2}}{\mu_{{off}\; 2}}} )}{2}$

where:

μ is the average value of pixel intensity in a given background regionin either a laser on or laser off frame as designated by the subscripts.

If the processor is not fast enough to keep up with the full frame rate,there needs to be a scheme to allow a random selection of frames to beprocessed Since n laser on and m laser off frames are used for thebackground cancellation, while waiting to accumulate this number offrames, any excess laser on or laser off frames can be discarded.

Alternatively a lock step synchronisation technique could be used sothat the computer was fed information about the state of the laser withrespect to the captured image. In any case, a minimum of one on frameand one off frame is required for the technique to work.

An alternative to the cancellation scheme described above is to simplysubtract laser on and laser off frames. Many on frames and off framescan be summed or averaged or low pass filtered, with the summing,averaging or filtering performed before and/or after the subtraction.

The result of the background cancellation is an image that ispredominantly composed of scattered light from the emitter, and someresidual background light and noise.

Frame Integration

At step 405 of FIG. 4 frame integration is performed. A number ofbackground cancelled frames are summed, averaged or otherwise low passfiltered to obtain a scattered light image with reduced noise. Byaveraging a number of frames, interference that is not correlated withthe laser on/off switching is reduced and the wanted (correlated)scattering information is retained. Typically the total number of framesused in the background cancellation and frame integration steps isapproximately 100 (i.e. approximately 3 seconds of video). Longerperiods of integration or lower filter cutoff frequencies may yield animproved signal to noise ratio, and allow a higher sensitivity system atthe expense of response time.

With reference to FIGS. 7 a to 7 c, the sequence of images shows theeffect of background cancellation and integration in the detection ofthe scattered light. The image intensity has been scaled to allow forbetter visibility to the eye. The particle obscuration level over theentire beam was approximately 0.15% per metre as measured by a VESDA™LaserPLUS™ detector, sold by the applicant. FIG. 7 a shows the rawvideo, FIG. 7 b highlights the region of integration, and FIG. 7 c thescattered light in the presence of smoke after background cancellationand integration.

Scatter vs Radius Computation

At step 406 of FIG. 4 computation of the scatter as a function of theradius from the emitter is performed. Variations in intensity along thebeam due to system geometry and scattering may be remedied using thismethod. A data array is calculated containing scattered light levels inthe integration region versus radius, for example measured in pixels inthe captured image, from the laser source. Since a radius arc covers anumber of pixels inside the integration, the intensity of each pixelwithin a given radius interval is summed together. FIG. 8 is a graphicalrepresentation of how the integration region is segmented by arcscentred with respect to the emitter. In FIG. 8, triangle 80 representsthe expected integration area and the arcs represent different radiifrom the laser source. Each portion of the integration area lyingbetween a pair of arcs has is pixels summed and the sum is entered intothe scattered light data array. For pixels that are not clearly betweentwo of the arcs, rounding or truncation of the calculated radiuscorresponding to such pixels can be used to resolve the ambiguity. Thecontribution of such pixels may also be apportioned to sumscorresponding to the adjacent areas, rather than being lumped into oneor the other.

Compute Geometry

At step 408 of FIG. 4, the geometry of the system elements/components isdetermined. Each pixel as described above (or image point) correspondsto a specific geometric configuration with respect to a scatteringvolume and the general case of such an image point is shown in FIG. 12.At each such point or pixel, the following parameters can therefore bedetermined:

1. θ—scattering angle.

2. r—the distance in meters from the laser source.

3. D—distance from camera to laser source.

4. L—physical length viewed by one pixel at a given point along thebeam.

A corrected intensity of pixels corresponding to a given radius, r, isthen determined for a real world system, in which the intensity ofpixels is multiplied by a predetermined scattering gain value, discussedbelow under Scattering Angle Correction, corresponding to the givenradius and a given scattering angle relative to a lossless isotropicscattering calculation. A resultant scattered data array is formed.

Scattering Angle Correction

A correction for scatter angle is logically determined in accordancewith step 409 of FIG. 4. As an input, the program requires a scatteringdata file, which contains for a given material, a set of scatteringangles and the corresponding gains. The data in this file is generatedby an empirical calibration process, and is intended to contain averagevalues for a variety of smoke types.

At each scattering angle as determined during the above geometrycomputation, the gain for every scattering angle is derived. The datafrom the input scattering data file is linearly interpolated so that forevery scattering angle an approximation of the forward gain can becalculated.

Compute Smoke vs Radius

A determination of smoke for a given radius of the beam is performed atstep 407 of FIG. 4. To convert the scattered data array to smoke levelson a per pixel basis requires input of data D, d and θ_(i) as shown inFIG. 12. Any combination of lengths or angles that constrain thegeometry can also be used. D is the distance from the camera 82 to theemitter 84, θ_(i) is the angle made between the line from camera 82 andthe emitter 84 and the line corresponding to the path of the radiationfrom the emitter 84, and d is the length of the line perpendicular tothe emitted radiation that intersects the camera entrance pupil. Fromthis information, all other necessary information can be determined bytrigonometry and geometry. The geometry can be seen in FIG. 12.

For each element in the previously described Scatter vs Radius array,the values of L, θ, and r, as shown in FIG. 12, are computed. L is thelength of the beam that is visible to one camera pixel.

Integrate Along Beam to Obtain Obscuration

At step 410 of FIG. 4, integration over beam image sectors is performedto obtain the detected obscuration. The beam length is divided into anumber of sectors to provide addressability along the beam. In order todistinguish between the laser source and scattering of the laser beam,the pixels around the laser source location cannot be included as partof a sector, as the intensity caused by scattering cannot be resolved,especially for an uncollimated source for which flaring may occurcausing residual intensity in the pixels surrounding the source.

Likewise at the camera end, due to the geometry of the set up, the fieldof view of the camera allows the beam to be viewed to within a fewmeters of the camera.

In order to provide a smooth transition between sector boundaries, asimple moving average filter is implemented. In fact, the beam isdivided into n+1 segments, and then a moving average is applied (oflength two segments) resulting in n sectors.

Each pixel along the beam-captured image corresponds to a physicallength along the beam see FIGS. 8 and 12. This physical length getssmaller as the beam approaches the camera. So starting at the laser endand ignoring the pixels that are outside the end boundaries, theobscuration for a particular sector is the sum of all the pixelintensities after the application of the correction noted above, whichfall into the physical length and position as described by that sector.

For example, to determine the obscuration, O, over the whole beam, givenas a sector size in pixel radius, r, as n to m,

$O = \frac{\sum\limits_{r = n}^{m}{{S(r)}{L(r)}}}{\sum\limits_{r = n}^{ni}{L(r)}}$

where S is scattered light and L is given above.

As noted above, the beam length is divided into a number of segments todetermine individual smoke levels for each segment effectivelysimulating a number of point detectors. The output of these notionalpoint detectors can be provided to a fire panel, which can then displaythe location of the smoke or fire as it would with normal point-typedetectors. The above formula is based on the theory that scattered lightemitted from each segment of the emitted radiation will provide adifferent light output for a given particle density based upon the anglefrom the radiation path to the camera and the number of pixels persegment. As the path of the emitted radiation comes closer to the camerathat is as r increases in FIG. 12 the angle θ_(r) increases. The numberof pixels that contain scattered light will also increase due to theapparent widening of the beam in the direction towards the camera 82.This increase in width is shown in FIG. 8 and FIG. 13, FIG. 13 shows theemitted radiation from emitter 84. The angle of the radiation spread isamplified for clarity. As the emitted radiation travels further from theemitter (that is as r increases), the number of pixels that coincidewith the location of potential scattered radiation increases. At theradius 86, close to the emitter, only two pixels are determined to bewithin the region of interest covered by the detector, and the lightfrom these pixels is summed and placed into an array 90, beingscattered_light(r), which comprises a n times 1 array of information,where n is the number of pixels across the screen. At radius 88, manymore pixels are within the area of interest covered by the detector, andthey are all summed to obtain the amount of scattering obtained withinthe covered region of interest. Calculated at array 92 is the scatteringradiation angle θ_(r), which will be different for each pixel. That is,when r is small, θ_(r) will be small, and as r increases, so does θ_(r).This information is important, as particles of interest in detectingcertain events can have different scattering characteristics based ontheir size. Very small particles (relative to the wavelength of theemitted radiation) scatter more uniformly regardless of θ_(r)(scattering angle), however larger particles scatter more in the forwarddirection, and reduce intensity as the angle θ_(r) increases. Quiteoften the particles of interest, in this example smoke particles, arerelatively small particles and therefore it can be useful to employ atable of effective scaling factors of output of light for givenscattering angles θ_(r). Such tables are known in the use of smokedetectors using laser chambers to detect particles.

Array 94 contains the actual radius of the light captured by the pixels.Array 96 comprises the length of the segment of the emitted radiationencompassed by, in this case, one horizontal pixel in the captured imagein the frame of the camera. This information is used to ascertain thevolume of the emitted radiation and is used to assist in the calculationof the radiation intensity. Also, array 96 contains data on the smokeintensity at each point r, defined as smoke [r].

Alarm State

Finally with reference to FIG. 4, alarm states are calculated. The alarmstates for each sector are determined based on thresholds and delays anda priority encoding scheme, as per standard aspirated smoke detectors,or other parameters determined by the user.

The same method is used for the zone alarm level, except that final noneoutput is the highest sector or the zone level, whichever is higher.

Fault Detection

The system may have provision for the detection of a fault condition,which is essentially the absence of the laser spot in the image. Thelaser on/off signal duty cycle may be checked to be within 33% to 66%over the number of frames used in one background cancellation cycle.

ALTERNATIVE EMBODIMENTS

A number of alternative embodiments are available, depending onapplication and desired features. For example, fault detection may becarried out in a number of ways.

In another application, the system described above could be used inapplications where measurement of obscuration was important, such asairports where fog may cause planes to divert if visibility falls belowa certain level. The system does not require ambient light to operate,and can therefore operate at night without additional lighting. Aninfrared camera could also be used with an infrared light source, wherethe light source, if of similar frequency to the detecting light, couldbe cycled so that the processor ignores frames illuminated for securitypurposes.

A typical security camera may take 25 images or frames per second. Smokedetection may only require detecting 1 frame per second or less.Therefore the remaining images can be used for security purposes.

To give increased sensitivity, video processing software operatingwithin the detection sub-system (6,7) may be used to eliminate thecontribution of nuisance changes in video signals which are not in thelocation known to be occupied by the light beam. Software based systemswhich perform a similar function of processing distinct areas of a videoimage are known, for example in video-based security systems such asVision Fire & Security Pty Ltd's ADPRO™ products.

The emitter may be a laser, emitting polarised radiation. The laser mayemit visible radiation, infrared radiation or ultra violet radiation.Selection of the wavelength of the radiation may be dependent on thecharacteristics of the particles to be detected, as well as thecharacteristics of the apparatus and method to be employed in thedetection of the particles. Other types of radiation emitter maycomprise a xenon flash tube, other gas discharge tubes, or a laser diodeor light emitting diode. The light is preferably collimated to at leastsome degree, but if the optional area segregation using regions ofinterest is employed, a broader radiation beam may be emitted.

A further embodiment is shown in FIG. 11 c, which employs two cameras102 and 104, and a single laser 106. In this embodiment, one camera canview the emitter, and the other the position or target where theradiation hits the wall 108. In such a configuration, it is desirable ifthe cameras 102, 104 are connected to the same processor or at leastcommunicate with each other. This system provides many advantages, suchas confirmation that the radiation is not blocked, and can be used todetermine more accurately a position of the emitter-radiation withrespect to camera 104, which detects the forward scatter of light. Assuch, the degree of uncertainty of the position of the path of theemitted radiation is reduced, and the regions of interest can be reducedin size, increasing the sensitivity of the detector system

In one aspect the present invention provides an apparatus and method ofdetecting particles comprising emitting at least one beam of radiationinto a monitored region and detecting a variation in images of theregion indicating the presence of the particles wherein the variation inimages is detected by at least one or more image capture devices, inparticular, use may be made of opposing cameras. More particularly, usemay be made of a pair of camera+laser pairs facing each other to:

-   -   Monitor each others laser source for integrity (correct        operation) and alignment    -   Make alignment easier in the case of infra red (IR) laser (use        camera to see the IR dot)    -   Obtain more uniform coverage in terms of sensitivity and        addressability    -   Obtain backscatter radiation

In the embodiment shown in FIG. 14 a first camera 31 and a second camera39 are mounted approximately opposite and facing one another with fieldsof view 320 and 310, respectively. Laser source 33 is mounted on thesame side as camera 39, and in one embodiment may be mounted on the samemount or in the same enclosure to give a cost benefit. Laser source 33and camera 39 may now be referred to as a “laser/camera pair” 33&39.

Similarly, a second laser source 311 is located on the same side ascamera 31 and may also be mounted on the same mount or in the sameenclosure to give a cost benefit. So, laser source 311 and camera 31also constitute a “laser/camera pair” 311&31. Laser sources 33 and 311provide laser beams 34 and 312, respectively.

Each laser and camera in a pair (33&39 or 311&31) may be pre-aligned atmanufacture so that the laser beam emerges at a fixed angle to thecentre of view of that camera. This provides the benefit that atinstallation time the mounting and alignment of each camera simplyinvolves directing the laser beam to point at an approximatepredetermined distance from the opposite camera, so reducinginstallation time and cost.

If the chosen pre-set angle is θ degrees and the separation between thelaser/camera pairs is D metres then the required target-spot to cameraseparation S is given byS=D tan θ

For example, if θ2 degrees, and D=50 m, then S is 1.75 metres. In suchan example, errors in positioning of for example, +/−100 mm would havean acceptably small effect on the particle density measurement accuracy.

A further benefit of this arrangement is that each laser beam arrivalspot indicated in FIG. 14 at 313 & 35 is in the field of view of theopposing camera and can therefore be readily monitored to ensure thatthe laser source is functioning correctly and the laser beam isunobstructed. This is an alternate form to the ‘rear-view’ mechanisms asdescribed elsewhere herein.

A further benefit of this arrangement is that it mitigates reducedpositional resolution which may be experienced when a single laser beamand one camera is used. In that case the precise position of a particlecloud that is distant from the camera may not be as accurately measuredas one that is close to the camera, since its image subtends a smallerangle and therefore fewer pixels in the camera image. With bio cameraspositional resolution is most accurate at each end of the protectedregion, and is reduced in the centre by only a much smaller amount. Afurther benefit of this arrangement is that it permits backscatter froma dense smoke plume to be readily measured. A further benefit of thisarrangement is that it facilitates the use of infrared light, since thecamera may be used to image the otherwise invisible target spot whenalignment is being performed.

In another aspect the present invention provides an apparatus and methodof detecting particles comprising emitting at least one beam ofradiation into a monitored region and detecting a variation in images ofthe region indicating the presence of the particles and furthercomprising means for determining the position of a given geometric pointin space within the monitored region.

It is useful to locate or find the source without the system being“dazzled”. In this respect, it is preferable to collimate the lasersource using a collimator or like construction with the purpose ofshielding the light scattered from the laser aperture from the receivingcamera as described elsewhere herein, and use a LED mounted on the lasersource to allow an opposing camera to locate the source. The LED may beflashed in synchronism with the laser and its intensity adjustedaccording to the ambient lighting, either by a photo-detector on thesource or by feedback from one of the cameras.

It would also be desirable for the system to be autonomously capable offully checking the optical geometry. This would involve determining the3D position of both the laser source and a target spot where the lasersource arrives relative to the camera location. The minimum requirementfor determining the position of a point in space using cameras may be tohave either two views from known vantage points of the point inquestion, or one view of the point and some other means of determiningdistance, such as angular separation. The physical distances may beprovided to the software of the system by the installer one way oranother as would be recognised by the person skilled in the art.

By locating another LED at a known position relative to the LED at thesource, the angular separation in the images can be measured and thedistance from the camera to the LEDs (and therefore the source) can becomputed.

Furthermore, by providing two ‘rear views’ the target spot position maybe determined. Thus the system is no longer at the mercy of impropersetting of the target spot distance or beam/camera distance. Fullyautomatic alignment may be possible.

In an alternate form, a camera may capture a view through two pieces ofglass or, partially silvered mirrors, or one thick piece. The result isthree images superimposed. The first image is the main image of thebeam. The second and third images contain the target spot on the walladjacent the camera. The two spots of the second and third image may notquite coincide. The centre-to-centre distance between the spots combinedwith the position of each spot in the image, and the known minor angleand spacing may be sufficient to compute the actual distance to thelaser spot in 3D space. The main image may contain some multiplereflections, so the source spot for instance may be seen more than once.The line containing these points provides information as to theorientation of the mirrors in terms of axial rotation. Thus the minorsmay be rotated (axially only) by the installer to allow the target spotto be to the left, right, above or below the camera without the need forany other mechanism to indicate the rotation.

Further description is now provided with reference to FIG. 15. In orderto accurately determine the location of any scattering point in theprotected space it is necessary for the system to internally model atany time the relative locations and orientations in 3 dimensions of thekey optical elements; being a laser source 43, a camera 41 and the laserbeam path 44 as shown in FIG. 15.

The laser beam path 44 may be resolved by determining the source 43location and any other point or, points along the beam, for example thearrival point of the beam indicated by target spot 46 in FIG. 15.

These positions may be determined manually by an installer and providedto the system through a man-machine interface using an arbitrarypre-defined coordinate system. However, it would be preferable for thesystem to be autonomously capable of fully determining the opticalgeometry, both for convenience of installation and for ongoing automaticverification that the elements of the system remain properly positionedand aligned. For clarity, the method is described here in reference toFIG. 15 with regard to a single source 43 and a single camera 41, butmay equally well be used for multiple sources and cameras.

To determine the source 43 angular position with respect to the camera41 in its simplest implementation, the light source 43 is in direct viewof the camera (indicated by angle 42 in FIG. 15) and the light sourceoutput, which may take the form of an aperture, a lens or a transparentwindow, emits enough off-axis light to allow the camera 41 to identifyits position in the image captured by the camera 41. This identificationis preferably facilitated by the modulation of the light source 43 in afashion, which permits image processing software to distinguish thelight source 43 from unwanted ambient light sources that do not havethis characteristic. However, in practise, it may be desirable that thelight source 43 is highly collimated and so there may not be enoughoff-axis light to allow this. The minimisation of this off-axis lightmay be deliberately arranged, using field-of-view masks and the like, asit is advantageous to prevent the saturation of the camera image in thisregion. Consequently, in order to make the position of the laser source43 distinguishable in the camera image, a further light source 46 with amuch less restricted emission pattern may be placed with the source 43or in close proximity to it. Preferably, a LED 46 of approximately thesame emission wavelength as the laser source 43 is used. The LED 46 maybe modulated in a fashion which permits image processing software todistinguish the LED emission from unwanted ambient light sources that donot have this characteristic, for example in its simplest implementationit may be flashed in synchronisation with the laser. Further, tominimise the effect of the LED light on the image, and also to minimiseany potential nuisance to people present in the room, the intensity ofthe LED 46 may be adjusted to the minimum level required. This may bevariable according to the ambient lighting, as measured by for example aphoto-detector at the source 43. Alternatively, the required LEDbrightness may be adjusted using software processing of image data fromone or more cameras 41.

By providing the source 43 with a further LED 47 at a known separationfrom the first LED 46 the angular spacing between these points may bedetermined from their respective positions in the camera image andsimple geometry may then be used to determine the distance between thecamera 41 and the light source 43.

Further, the two LEDs 46, 47 are positioned at a known verticalposition, for example, preferably each LED 46, 47 is installed at thesame height so that a line drawn between them is horizontal such thatthe angular tilt (yaw) of the camera 41 may also be determined.

Having established the relative location of the beam source 43 withrespect to the camera 41 it is necessary to determine the beam path 44.One or more of the following methods may achieve this:

a) causing the target spot 45 to fall within the direct view of thecamera 41;

b) manually or automatically placing a partially scattering medium inthe path of the beam 44, either permanently, or as and when it isrequired, to check the beam position;

c) detecting and recording the scattering caused by airborne dust motes(small particles) that occasionally fall within the beam 44;

d) using a reflecting or refracting device to enable the camera 41 toview a target spot 45 that falls outside its direct field of view;

e) using a further imaging device to monitor the target spot 45position.

Alternately as noted above, by providing two ‘rear views’ the targetspot 45 position may be determined.

In preferred forms described herein, the present invention provides amethod and apparatus for synchronisation between a light source and acamera comprising allowing the source to oscillate on and off at apre-determined rate, identifying the video image of the source in thecamera and then continually modifying the camera frame rate to remain insynchronisation. This has the benefit of reducing cost, for example, ofwiring or radio communication between the source and camera. This mayalso allow for a low cost powering means for the system such that remotepositioning of the components is viable by way of using internal batterybackup on lasers remote from cameras. Normal power for the laser may beprovided from a plug pack or other low cost supply. In other words, apair of AA NiCad batteries may be sufficient. The battery backed powersupply should be such as to conform with the requirements for Firesafety systems ie UL approved power supply for fire

In one particular embodiment the source may fitted with a secondarylight source with a wide angle of emission, such as an LED as describedwith reference to FIG. 15. The LED may flash in synchronisation with thelaser light source to facilitate the location of the source in thecamera image. Equally, the LED may be turned on and off autonomously,with the camera synchronising to it. While on backup power, the lasercould drop the duty cycle to indicate the condition and also to conservepower.

In a preferred embodiment the camera frame rate may be initiallycontrolled to free-run at approximately the same rate as a free-runningoscillator in the source. When the flashing source or LED image issubsequently identified, the rate of change of phase between the twooscillators may b identified and a conventional Phase-Locked-Loopfeedback method may then be used to adjust the camera frame rate tomaintain a fixed phase and so remain in the required synchronisation.Other status info may also be transmitted via the laser modulation or byadditional LEDS.

In another embodiment, the source may be arranged to flash not in asimple periodic on-off pattern but in a more complex, yet predictable,pattern such as a pseudo-random sequence. This permits the source to bemore readily distinguished from other nuisance light sources, such asfluorescent lights, which vary in a periodic manner uncorrelated withthe source modulation. This benefits both in making initial location ofthe source in the video image easier and in improving the sensitivity tosmoke in the presence of varying ambient light.

In yet another embodiment, the primary frequency of the sourceoscillator may be altered to be at, or a multiple of, or a sub-multipleof the AC mains electricity frequency (normally 50 Hz or 60 Hz dependingon the region) and is synchronised in phase to it. The mains frequencymay be sensed directly by a wired input from the mains supply, or may besensed by an inductive or capacitive coupling, or alternatively may besensed by a photo-electric detector receiving light from the artificiallighting in the area. Where there is no artificial lighting, then theoscillator may run freely at its default frequency without loss ofbenefit. In a further embodiment, the primary frequency is set to afrequency very near to that of the AC mains electricity frequency, or amultiple or submultiple, but no synchronisation means is provided.

In another preferred aspect the present invention provides an apparatusand method of detecting particles comprising emitting a beam ofradiation into a monitored region and detecting a variation in images ofthe region indicating the presence of the particles wherein thevariation in images corresponds to backscattered radiation. Inparticular, a second laser source may be mounted on a camera, fordetecting images of the region, at an angle so that the emitted beamcrosses the field of view of the camera, so looking for backscatter atan angle slightly less than 180 degrees to beam direction. In thisrespect it may be possible to detect backscattered radiation to measurelevels of particles that may totally obscure an incoming beam from thefirst distant laser source. In cases of high smoke levels a beam may betotally obscured from view at a location opposite the smoke event. Thisaspect is described in more detail elsewhere herein.

Backscatter Geometry

For the purposes of this description “backscatter geometry” may be anarrangement where the scattering angle is greater than 90 degrees. Thescattered light may therefore be heading back in the general directionof the source. An embodiment of a backscatter system may have the laser(or other electromagnetic source) built into the same housing as thecamera, or alternatively mounted nearby the camera. In a backscattersystem, the camera may generally receive much less scattered light thanin an otherwise similar forward scatter arrangement. So such a system isgenerally not preferred for high sensitivity since additional measuresmay need to be taken to achieve the same performance level as forwardscatter detection. However, the backscatter arrangement offers certainadvantages, both when used alone and as an adjunct to a forward scattersystem.

With reference to FIG. 16, there is shown the general physicalarrangement comprising a laser 162, camera 101, laser target 164 onwhich the laser beam may form a target spot 163, and additional lightsources 165 as markers. Note that while FIG. 16 shows a number ofelements to obtain a specific benefit or function listed below, not allof these elements need exist. The camera 161 and laser 162 may bemounted in the same housing or at least in close proximity. This allowseasy installation since there is no need for wiring or power or signals,apart from the light beam 166 itself, to connect the ends of the spacebeing monitored 167. A forward scatter system would require some meansof powering the light source 162 that is remote from the camera 161, andalso of synchronising the light source 162 to the camera shutter (wherecorrelation techniques are used). A forward scatter system could use aminor at the far end to allow the laser to be near to the camera, but inthis case alignment would be much more critical.

If the laser 162 and camera 161 are either mounted together in the sameframe or housing, then they could be factory aligned. This makesinstallation easier since the installer only has to set the laser'svisible spot 163 to fall on the desired target area 164, and the camera161 will be correctly aligned. If invisible radiation is used, then amonitor showing the image from the camera 161 may be used to assistalignment. The monitor could be a computer with monitor screen connectedvia a digital data or network connection.

In the case of the factory aligned camera 161 and laser 162 it is notnecessary for, the distance across the space to be known since the laservideo system may measure it itself using the same geometrical techniquesas is used for determining the position of smoke particles. Essentiallythe approach would be to find the laser target spot 163 in the image,using techniques already described, and then convert this imagecoordinate into a spatial coordinate using the geometric models alreadydescribed. The advantage here is that there is one less task for theinstaller to do, or that the installer entered data can be verified bythe system.

In any smoke detection system it is desirable to monitor all functionsfor fault conditions so that the system can be properly maintained. Abackscatter system may have the light source target spot 163 in view ofthe camera 161. Therefore monitoring of the integrity of the lightsource 162 both in terms of its operation and also for external blockagecan be achieved at low cost. The software for determining the presenceand position of the spot 163 is likely to be present for reasonsmentioned earlier.

A backscatter system is very tolerant of misalignment or alignmentdrift. This is particularly so if the camera 161 and light source 162are mounted in the same housing. In fact, it is so tolerant that it maynot detect that the cameral/laser 161,162 unit has been swung to pointin a completely different direction and is thus no longer covering theintended area. There are some techniques for detecting this condition.They are:

1) Use edge detection and correlation to determine whether the scene issubstantially the same as when it was originally installed, and raise afault if it is not.

2) Use a target 164 that is easily recognised using image-processingtechniques such as a cross and if the position of the target marker 164(within the image) moves by more than a threshold amount since the timeof installation a fault is raised.

3) Use an additional light source or sources 165 within the field toprovide markers. The use of more than one marker 164 allows positivedetection of camera rotation. These sources 165 could be synchronisedwith the main light source 162 to simplify processing. If the positionof the source or sources within the image move by more than a thresholdamount since the time of installation a fault is raised. Theseadditional light sources 165 could be mounted on other laser/camera162,161 units mounted in the same general area, thus eliminating theneed for extra wiring. The light sources 165 could also be sources thatare present primarily for the purpose of particle detection inconjunction with the camera in question or any other camera.

In any scattering based detection system the scattered light isattenuated by further scattering and absorption by interveningparticles. In the case of a forward scatter system using shallow scatterangles the path length is substantially the same wherever the scatteringoccurs. So when the concentration of particulate exceeds a certain valuethe amount of scattered light received at the camera 161 begins to fall.So forward scatter systems may need an alternative means of measuringparticle concentration that is used when high concentrations areexperienced. The backscatter system may be used for this purpose sincethe path length is roughly proportional to the distance from the camera161 and laser 162. Even when the particle concentration is very high,the scattering that occurs close to the camera 161 can still bereceived.

In addition to the above, a path loss measurement can be made byobserving the intensity of the target spot 163 in the image, compared tothe value recorded at the time of installation. This data can be usedalone to estimate the average particulate density. The data from thismay also be used in conjunction with the scatter information and somecorrections to estimate the mean density within segments of the beam 186despite the particle density having exceeded the turnover pointdiscussed above.

The data from these measurements may also be used in conjunction withthe scatter information to discriminate between smoke types and nuisanceparticles. The technique comprises computing the ratio of scatter toattenuation and comparing this against ratios for known materials. Thiscan be done for the whole beam 166 and also for segments of the beam166.

In a system comprised of two camera/laser pairs, most of the abovebenefits can be obtained while maintaining the sensitivity benefits offorward scatter.

In another preferred aspect the present invention provides an apparatusand method of detecting particles comprising emitting a beam ofradiation into a monitored region and detecting a variation in images ofthe region indicating the presence of the particles and furthercomprising at least one additional beam adjacent the beam for detectingan imminent intrusion into the beam.

Lasers of sufficient power can cause eye damage and even skin damage. Itwould be undesirable for a system of the present invention to present asafety hazard. The simplest approach to having a safe system is to keepthe laser power sufficiently low, but this may compromise thesensitivity of the system. An alternative is to have a scheme to switchthe laser off or to a safe power level whenever there is a risk ofexposing human tissue or the like. It is also important that such asystem does not switch the laser off unnecessarily since continuity ofsmoke detector operation is also important.

FIG. 17 shows a representation of a laser interlock system where a highpower laser 171 is inside a ring of low power eye-safe lasers 172. Theouter beams are spaced sufficiently closely (eg 100 mm apart) so that itis impossible for a human eye to be subjected to the laser beam from themain laser 171 without first having blocked one or more of the outerbeams. A camera 173 senses the laser light scattered from the target175. Processing hardware and software 174, process the images todetermine the presence or absence of the target spots 176 correspondingto the outer lasers. If one or more of these target spots are absent theprocessing hardware and software turns off the high power laser. Thehigh power laser is not allowed to operate again until all of the targetspots are present in the image.

The spacing of the outer beams is chosen so that at the highest expectedvelocity of a human head there is insufficient time for the eye to reachthe main beam before the camera and processing system has detected theintrusion and turned off the main beam.

Background Cancellation

Techniques for reducing the effects of background light as alreadydescribed can be used to enhance the image of the target spots 176. Theouter beams may need to be switched on and off for these techniques towork. Depending on the delays in the camera and image acquisition andprocessing system it is possible to reduce the response time byoperating the outer beams in the opposite phase to the main beam.

Alternatively, the outer beams may be left on most of the time, withonly occasional image frames taken with the outer lasers off for use inbackground cancellation processing. If the interlock response time istoo long at the time these off frames are being acquired, then the mainlaser 171 may also be disabled during these periods.

Active Target

Instead of using a camera to collect an image of the target, the targetmay have photo-detectors mounted on it. Such detectors may already bepresent for the purposes of maintaining or monitoring system alignment.

Shorter interlock response delays are possible with this arrangementsince the camera frame rate limitations are removed.

Cage Propagation Direction

The outer lasers beams do not need to propagate in the same direction asthe main laser. These laser sources could be mounted around the mainlaser target, and propagate towards the main laser source, landing ontargets points around the main laser source. The advantage of thisarrangement is that the same camera that is used to detect forwardscatter from particles in the main beam can also be used to captureimages of the outer laser beam target spots.

Camera Laser Pair Configuration

A pair of cameras and lasers can be arranged to provide mutualsupervision as described elsewhere herein. In this case they can alsoperform the image collection, processing and main laser control for theinterlock function.

Tube

The protective beams could be a tube of light rather than separatebeams. Such a tube of light would appear as, for example, a circle orellipse at the target. The image processing software would then need todetect interruptions or shadows in the expected ellipse. There areseveral image processing techniques that could be used as would beappreciated by the person skilled in the art.

Note that the tube does not have to be circular, and it does not evenhave to be hollow. A solid cylinder will also work. The expected targetshape will then be a filled circle, ellipse or other shape. Again, theimage processing software would then need to detect interruptions orshadows in the expected ellipse.

Hologram

An interference grating or hologram can be used to create the outer beamor beams from a single laser source. The single laser source could bethe main laser 171, or an independent laser,

Virtual Cage

An alternative to an actual cage of lasers is to use a ring of lightsources (that are not necessarily tightly collimated) around the mainlaser. A camera mounted at the main beam target near the axis of themain beam views the light sources. For an intrusion to enter the mainbeam it must first block the camera view of the outer light sources.Similar processing to that required in the previous arrangements canthen provide the interlock function.

Video Motion Detection

In another embodiment image processing techniques such as video motiondetection used in security products sold by Vision Fire and Security PtyLtd may be used to detect an object, such as a person, approaching tooclosely to the hazardous laser beam. The signal from this may be used toswitch off the beam or reduce the laser power to eye safe levels. Thistechnique may not be applicable in darkness, but is nonetheless usefulsince a high power laser would not generally be required when theambient lighting is low.

In another preferred aspect the present invention provides an apparatusand method of detecting particles comprising emitting a beam ofradiation into a monitored region and detecting a variation in images ofthe region indicating the presence of the particles wherein at least oneof the beam of radiation and a means of detecting the variation inimages is adapted to communicate data.

In most fire protection systems, the installation of wiring is asignificant contributor to the total system cost. Wireless systems basedon radio communication equipment dedicated to data communicationsprovide an added cost to systems.

The following is a list of some of the data that may need to becommunicated between different parts of a detector system in accordancewith embodiments of the present invention:

1. Camera/Laser synchronisation or timing information

2. System Configuration Data

3. Laser intensity, duty cycle and camera exposure commands

4. Laser & camera alignment data (for active alignment and/or faultmonitoring)

5. Laser enable/disable commands in multi-laser systems

6. Laser marker activation/de-activation/duty cycle/intensity controlcommands

7. Laser polarisation or wavelength switching commands

8. Fire Alarm status for reporting to the fire panel or other externalsystems

Optical communication may be used in conjunction with radio basedcommunication to improve the overall communication reliability.

In accordance with a preferred form of the invention, the opticaltransmission of the emitted radiation source(s) may provide a datacommunications path between all of the data processors within an area tobe protected. One of the processors may act as a point for connection toexternal systems, such as a fire alarm panel. Optionally, two or moresuch points having independent data paths to the fire alarm panel may beused to provide a fault tolerant reporting path.

The exact needs of a specific system will depend greatly on thecomplexity and type of system configuration. However, in general,available bandwidth should be the measure used to distinguish theclasses of communications solutions and their utility. Solutions thatuse a camera will be bandwidth limited in some way by the frame rate ofthe camera, while solutions that use some other photo-sensor will nothave this limitation and so should, in principle, be capable of higherbandwidth.

In FIG. 18 there is shown an example system comprised of two cameras 181a & 181 b and two lasers 182 a & 182 b arranged to allow mutualmonitoring of the integrity of the laser beams. This concept isdiscussed more fully elsewhere herein, Two photo-detectors 183 a & 183 bconvert a portion of the incident laser signal into an electricalsignal. The received signals are passed to processing electronics withinor associated with the cameras 181, which in turn generate controlsignals that are fed to the lasers 182. To reduce the effect ofbackground light, the photo-detectors 183 may employ an optical bandpass filter such as an interference filter or a coloured dye filter. Apolarising filter may also be used if the laser is polarised. Linear orcircular polarisation may also be used.

The main sources of interference can be expected to be of a DC or lowfrequency nature from sunlight or man-made light sources. An approach todealing with this sort of interference is to frequency-shift the dataaway from the frequencies where interference exists, in this caseupwards. FIG. 19 shows such an arrangement where the processor 191 feedsdata to a modulator 192 which in turn feeds the frequency shifted datato the laser 193. The laser 193 generates amplitude modulated laserlight 194 in accordance with the signal from the modulator 192.Photo-detector 195 converts the received light back into an electricalsignal, which is then sent to demodulator 196 before being passed toprocessor 197.

Many modulation techniques may be employed in accordance with thepresent invention. Some examples are given below.

One approach is to amplitude-modulate the laser with the serial datastream. If the background light levels are low enough then this willwork. The statistics of the data stream may cause some variation in theaverage laser power. This in turn will affect the sensitivity of thesystem, although since the effect is calculable it could be correctedfor. The data can be encoded to reduce or eliminate the variation in theaverage power. For example the data can be randomised by an “exclusiveor” operation with a pseudo-random sequence. Data compression techniquescan also be used since they will tend to randomise the transmitted datastream.

Another scheme is Manchester encoding, since it results in a constantaverage power and no DC data content.

Pulse position modulation may be used. In this case the pulses could beshort periods where the laser is switched off or to a lower powers withmuch longer intervals in between at the full power. Such a scheme offersnear constant average power, and a higher average power to peak powerratio than Manchester encoding.

Pulse width modulation could also be used. Again the case the pulsescould be short periods where the laser is switched off or to a lowerpower, but rather than varying the position in time, the duration orwidth is varied. Provided that the pulses are short compared to the timein between, then the average power to peak power ratio will be high andthe variation in the average will be low. The data can be encoded toreduce or eliminate the variation in the average power. For example thedata can be randomised by exclusive or with a pseudo-random sequence orit could be Manchester encoded prior to the pulse width modulator. Avariation on pulse width modulation would use absent pulses instead of anon-zero width. In this case the absence of a pulse at the expected timerepresents a specific data symbol in the same way as a pulse of aparticular width represents a specific, but different data symbol.

Also, many of the above techniques can be combined, and some othertechniques that could be employed are sub-carrier with frequency shiftkeying, subcarrier with phase shift keying, sub-carrier with amplitudeshift keying and spread spectrum techniques.

Since a camera may only give an update of the light level falling on apixel once per frame, the data rate is limited by the frame rate. Thiswould imply a rate of only 30 bits per second with a frame rate of 30Hz. However, there are techniques that may be used to increase the datarate beyond one bit per frame.

Ambient lighting is a noise source that may interfere with the datacommunications. Optical filtering as previously described can beemployed. Since the camera is primarily present for smoke detectionpurposes, the filters are likely to be already present.

The methods already described for minimising the effects of backgroundlighting on smoke detection performance are also generally alsoapplicable to data reception, and will not be discussed further here.

Many of the modulation or encoding schemes discussed in the previoussection can also be used in the case that the receiver is a camera. Inorder to mitigate frame rate imposed limitations, data compression isparticularly desirable.

Since most cameras will integrate the received photons over a definedexposure period, the emitter duty cycle during the exposure period canbe varied to get same result as varying the actual intensity. In somecases this will be a lower cost implementation.

A method that makes use of hardware already present in the example ofFIG. 18 is to modulate the intensity of the laser with the data. Thelaser must be visible within the field of view of the camera, and musthave sufficient output directed towards the camera for it to overcomethe background lighting variations. These conditions should already bemet in many embodiments of the invention as part of the laser beamintegrity monitoring.

There are many methods for encoding the data and some examples follow.For the sake of explanation it is assumed that in an system of theinvention that does not send data via the laser, the laser is simplydriven on and off in alternate frames. The transmission of data is thenjust a matter of varying the pattern of on and off periods or frames,and/or varying the intensity, and then identifying the variation at thereceiving-end camera and processor.

Following a synchronising sequence, the regular laser on-off drive canbe exclusive or-ed with the data stream before being applied to thelaser modulator. One bit is transmitted per two frames in this method.This method can be regarded as a form of Manchester encoding. The mainadvantages of this method are simplicity, robustness, and that theaverage duty cycle of the laser is unchanged by the data. The data rateis however very low. Data compression methods may help recover somebandwidth.

A higher data rate may be achieved by transmitting one bit per frame byapplying the data stream directly to the laser modulator. In order tokeep the average laser duty cycle within acceptable limits for smokedetection operation, some means are still required limit long runs ofthe same symbol. Again randomising and/or compression techniques may beused.

It is also possible to increase the date rate further by usingmulti-level encoding. For example for different laser intensities suchas a %, 33%, 66% and 100% of full power could be used to encode two bitsper frame. External optical noise, camera internal noise, and overallsystem gain stability will limit the number of levels that can be used.

Instead of using the laser alone as the data transmitter, additionallight sources that can be modulated may be used, as represented by items184 a and 184 b in FIG. 18. If optical filtering is used at the camera,the light source(s) chosen must emit at the corresponding wavelength andpolarisation, or be of sufficient intensity to overcome the filterlosses. Light emitting diodes (LED) are well suited to the purpose.

For example, an array of 4 LEDs can transmit 4 times as much data as onelight source alone. Any of the preceding methods for data encoding canbe applied. The LEDs must be sufficiently spaced so that they can bedistinguished at the camera as individual sources.

If a colour camera is employed, then the camera can measure theintensity of up to three differently coloured LEDs, even if they appearat the same point in the image. Three separate LEDs can be used, or anRGB LED can be used. In this way, 3 bits of data can be transferred perframe per RGB LED. Again, any of the preceding methods for data encodingcan be applied. For example, a four-level encoding scheme with one RGBLED could be used to transfer 6 bits per frame.

In another preferred aspect the present invention provides an apparatusand method of detecting particles comprising emitting a beam ofradiation into a monitored region and detecting a variation in images ofthe region indicating the presence of the particles further comprisingmeans for compensating for distortions in the detected images.

With reference to detecting images, most lenses will produce some degreeof image distortion compared to a pinhole lens. Typically the distortionis a radial magnification around a distortion centre that usually isclose to, but not necessarily exactly on the optical centre line. Thedistortion is often termed “pincushion” or “barrel” distortion dependingon whether the magnification increases or decreases with radius from thedistortion centre.

Lenses with narrow fields of view, for example less than 20 degrees donot generally produce enough distortion to significantly affect theoperation of an image capturing device for the purposes of detectingparticles. However, wide field of view lenses may produce sufficientdistortion that a particle detection system may not operate correctlywithout some measures being taken to combat the distortion.

Dealing with Lens Distortion

If no attempts are made to correct lens distortion in a particledetection system that uses emitted radiation and image detection inaccordance with the present invention, the following effects may occur.

1. Integration Region. The integration region may not properly coincidewith the actual position of the beam in the image, since the beam isassumed to be straight, but may actually appear curved.

2. Spatial Accuracy: The computed position in space that corresponds toa particular pixel may be in error.

3. Gain Error: The length of beam that corresponds to a particular pixelmay be in error, resulting in a system gain error.

In accordance with a preferred form of the invention, the followingtechniques may be used to combat some or all of the above effects.

Low Distortion Lens

For a given field of view, a compound lens design can be optimised togive less distortion. With suitable lenses, systems requiring only anarrow field of view may not need any corrections for lens distortion.

Empirical Spatial Calibration

An empirical calibration of the relationship between points in the imageand points in space can be performed. This can be done by placing anobject that causes some scattering in the beam, and then recording theposition of the object in space and also the position as it appears inthe image. This process is then repeated for a number of points alongthe beam.

This empirical calibration can be performed with a device describedelsewhere herein as a “commissioning device”. Such a device willprobably be necessary for the purpose of testing installed systems forcorrect alignment. In its simplest form it would comprise of a piece ofmaterial that scatters some part of the impinging radiation (such as apiece of transparent plastic or glass) mounted on a stick to allow it tobe easily place in the beam by an operator or installer.

The minimum number of points required will depend on the degree ofdistortion and the typo of interpolation subsequently used. Points at ornear each end of the active portion of the beam should ideally beincluded. An option is to record a point at the boundary of eachintended sector. Each sector may behave as a separate “virtual”detector, with its own alarm logic etc.

The recorded data may then be used in the following ways.

1. The integration area is chosen to include the recorded points.Interpolation or curve fitting is used to estimate the requiredintegration area between the points. The integration area is madesufficiently wide at each point to allow for the beam divergence and anyuncertainty in the position of the two points.

2. The recorded points can be used as a lookup table to determine theactual spatial position corresponding to a given pixel or group ofpixels. Interpolation is used to estimate values that fall in betweenthe recorded points, or alternatively if the recorded points are theboundaries of each sector, then it is sufficient to use this data todetermine which sector each pixel belongs to for use in subsequentreceived scattered light integration operations.

These methods may address the first two effects mentioned.

The third effect of gain error can either be ignored, since in manycases it will be a relatively small error, or for example by calibratingthe camera with a uniformly illuminated scene. This type of calibrationor correction may also be needed to correct for other sources of gainerror such as camera vignetting anyway it is worth noting that this sortof correction will be correct for those parts of the image where thelaser beam subtends at least one pixel in width, however where the beamis very narrow the correction may be less accurate because the beam is aline source rather than a surface—which was the basis of the correction.

Laser Beam Orientation

The laser beam and camera can be aligned so that the image of the beampasses near the image centre. Since distortion is mostly radial theresult will be that the beam still appears as line. This is a measurethat allows the integration area to be calculated in a way fromknowledge of only two points along the beam by drawing a straight linebetween the points, and sufficient width to allow for the beamdivergence and any uncertainty in the position of the two points.

Model Based Distortion Correction

Modelling

A mathematical model can be used to represent lens distortion. In mostcases a radial distortion model is sufficiently accurate. An example ofsuch a model isr′=M(|r|)·r

where:

r is a vector representing the true position of a pixel,

r′ is the distorted position of the pixel and

M is a scaler magnification factor that is a function of the distance ofthe pixel from the distortion centre, and constrained such that M(0)=1

The vector distances are all measured with respect to a pointP=(P_(x),P_(y)) that represents the centre of distortion of the lenssystem.

The model represents a mapping between the distorted image plane and theundistorted image plane.

Various methods for arriving at the function M for a given lens arediscussed in literature that would be available to the person skilled inthe art.

One approach is to:

Let M(r)=1+ar+br² (or use a higher/lower order polynomials; forimproved/reduced accuracy)

Record an image of a scene composed of a uniform array of black dots ona white background

Choose one or more rows of dots

Determine the coordinates of their apparent centres in the image (whichis the distorted image plane).

Use a least squares optimisation to determine the best-fit coefficientsa, b, P_(x) and P_(y) that make the points fall as nearly as possible toa straight line (or lines if more than one row was chosen) when mappedto the undistorted image plane.

This modelling may be carried out at least for each type of lens that isused in a system according to preferred forms of the invention, orpreferably for each individual lens at the time of manufacture of thecamera unit. The model coefficients are then stored permanently in anassociated processing unit or non-volatile memory physically associatedwith the camera. Other camera related calibrations could be dealt withsimilarly, for example fixed pattern noise correction data andpixel-by-pixel sensitivity data can factory measured and stored in thecamera unit or associated processor.

Correction

The distortion model can be used in several ways. Conceptually one wayis to fully “un-distort” entire images as the first processing stepafter capturing them from the camera.

One method is to set each pixel value (grey level) in the resulting“un-distorted image” to the value of the nearest corresponding point inthe original distorted image, using the known model to convert thecoordinates.

Since the pixel coordinates after the mapping into the distorted imageplane is often fractional, a more accurate method is to useinterpolation to obtain an approximation of the pixel value. Bilinearinterpolation yields good results, but a full sinc(x) interpolation maybe more useful.

Correcting the whole image is computationally intensive, so it isadvantageous to use methods that avoid correcting the entire image.

The preferred method is to do all of the processing as previouslydescribed, and apply corrections at the following points in theprocessing sequence:

1. When computing the integration area, un-distort the coordinates ofknown points (e.g. laser source spot, target spot if visible, memorisedimages points obtained with the commissioning device)

2. Compute a set of pixels within an enclosing polygon that makesallowance for beam divergence and uncertainty in the position of thepoints (same as would be done if there were no lens distortion).

3. Map the coordinates of each of the pixels back to the nearest pixelposition in the distorted image plane,

4. Repeat above steps for the background cancellation areas

5. All coordinates used in computing the “pixel radius” (distance of apixel from the apparent position of source in image) should be firstmapped to the undistorted image plane.

6. Similarly, coordinates used in computing all geometry relatedquantities (scatter angles, corresponding position on laser beam 8)should first be mapped to the undistorted image plane.

In this way the integration area takes correct account of the lensdistortion, and appropriate corrections are also made for scatteringangles and also spatial positions of particles, without the verycomputationally intensive process of fully correcting entire images.

Note that it may still be desirable for the system of the presentinvention to be able to correct entire images on occasion for:

1. Visual verification of the distortion model,

2. Delivery of surveillance images to external systems.

In another preferred aspect the present invention provides an apparatusand method of detecting particles comprising emitting a beam ofradiation into a monitored region and detecting a variation in images ofthe region indicating the presence of the particles further comprisingmeans for providing a weighting function to detected images forselectively resolving image portions.

The resolution of the camera limits the resolution or accuracy of themeasured position of detected particles. In a system using forwardscatter geometry, the position of particles in the beam that are nearthe camera may be resolved to a high accuracy, however for particlesthat are more distant the resolution becomes increasingly worse

In FIG. 20, source 201 directs a beam of light in the direction of thecamera a camera composed of lens 202 and photosensitive surface 203 suchthat forward scatter from particles in the beam can enter the camera.The fields of view of two camera pixels are represented by the angles θ1and θ2. These angles approximately are the same for ordinary lenses. Theportions of the beam that are visible in the fields of view of the twopixels are represented by ΔL1 and ΔL1. Even without any calculations itis clear that the length of the beam that corresponds to a single pixelincreases dramatically as the distance from the camera is increased. Tofirst approximation, the length ΔL is proportional to the square of thedistance of the beam portion to the camera.

In practical terms this means that the minimum required cameraresolution for a system is set by the desired performance fordetermining the position of particles at the far end of the beam. As aconsequence there may be far better performance than is needed at thenear end.

A lower resolution camera may be used to achieve a given systemperformance by using a lens system that caused the pixel fields of viewto be wider for the pixels viewing nearby parts of the beam, andnarrower for those viewing distant parts. Note that when deliberatelydistorting optics is used, image-processing corrections as describedelsewhere herein will generally need to be applied to maintain thecorrect system operation. The deliberately distorted optics describedhere cannot be modelled using a simple radial distortion model as isoften done for ordinary lenses, however apart from determining thecorrect distortion model to use, the processing that deals with lensdistortion can be the same as that described hereinabove on lensdistortion. A compound model may be used in this case a combination of aradial distortion model and a prism model may be used.

Offset Lens

One technique is known as an offset lens. In FIG. 21, a camera composedof lens 212 and photosensitive surface 213 senses scattered lightoriginating from light source 211. The lens is offset from the centre ofthe photosensitive surface, and may possibly be also tilted to reduceaberration in the image. The photosensitive surface is arranged to beapproximately parallel to the light beam.

Prism

Another way to achieve a similar effect is to use a prism. An example isshown in FIG. 22 where a camera composed of lens 222 and photosensitivesurface 223 senses scattered light originating from light source 221.The scattered light passes through prism 4 before entering the lens. Theeffect of the prism is to expand or compress the angular subtense oflengths of the beam in a manner that varies depending on the angle ofentry to the prism. Prisms with curved surfaces can be also be used toobtain more exaggerated effects than flat-sided prisms. Multiple prismscan also be used to increase the effect.

Curved Mirror

A further method uses a curved mirror. An example is shown in FIG. 23where a camera composed of lens 232 and photosensitive surface 233senses scattered light originating from light source 231. The scatteredlight is first reflected by curved mirror 234 before entering the lens.The effect of the curved mirror is to expand or compress the angularsubtense of lengths of the beam in a manner that varies depending on theangle of incidence to the mirror. Although a convex mirror is shown,concave mirrors or mirrors with convex and concave parts may be used.Generally the mirror would be singly curved, although a doubly curvedmirror can also be used.

In another preferred aspect the present invention provides an apparatusand method of detecting particles comprising emitting a plurality ofbeams of radiation into a monitored region and detecting a variation inimages of the region indicating the presence of the particles whereinthe beams are adapted to be sequenced in operation.

In a preferred form, the present invention may be made to cover a largerarea by adding extra laser beams. If all the beams and the camera lieapproximately in a plane, then the beams will approximately overlap fromthe perspective of the camera. This may result in both a systemsensitivity improvement as well as an increase in the area covered bythe one camera. The multiple lasers provide a similar sensitivityimprovement as an increase in laser power since the camera & backgroundnoise contributions are substantially the same as for one beam.

In some forms, it may not be necessary to isolate the position of theparticulate down to a single laser beam. However, if required it isstill possible to tell where the particulate is located by cycling thelaser beams on and off.

A scheme adapted to provide this result would have all the lasersoperating one frame on and one frame off as would be done with a singlelaser. When particulate is detected, the system can then switch to ascanning mode where only one beam is on at a time.

A more elaborate scheme that allows a higher average power while“scanning” is as follows: Every second frame has all the lasers off,while in every other frame all but one laser would operate. In each“laser on” frame a different laser is not operated. Any other linearlyindependent combinations of laser state could be used. Also, variedlaser powers can be used rather than completely turning the lasers off.However, the scheme described here is preferred for its simplicity andthe high laser duty cycle that is achieved. Note that lower duty cyclesmay be preferred in some cases to reduce power consumption and increaselaser life.

In FIG. 24, camera 241 receives scattered light from laser beamsgenerated by the lasers L1, L2, L3 & L4. The camera field of view is θ.The field of view corresponding to a pixel is Δθ.

The timing diagram of FIG. 24 shows the pattern of operation of thelasers. As noted hereinabove, other patterns can also be used.

The mathematics for converting the camera signals into separatescattering readings for each beam is as follows:

Let:

R be the total received signal at one pixel in the image from thecamera,

S_(n) be the contribution from particles illuminated by laser n whenlaser n is at full power.

L_(n) be the power of the n^(th) laser where 1 represents full power,and 0 represents a laser off state. (Fractional laser powers 0<L_(n)<1also allowed.)

N be the total number of lasers

Then,

$R = {\sum\limits_{n = 1}^{N}{L_{n}S_{n}}}$

Now if N frames are taken, each with N linearly independent vectorslaser states [L₁₁ . . . L_(1N)] . . . [LN_(N1) . . . L_(1N)] and weassume that the scattering contributions that we seek [S₁₁ . . . S_(1N)]. . . [S_(N1) . . . S_(1N)] are constant over the period that the datais collected (i.e. [S_(m1) . . . S_(mN)]=[S₁ . . . S_(N)] for 1≦m≦N),then the corresponding received signals R_(m) will be:

$R_{m} = {\sum\limits_{n = 1}^{N}{L_{mn}S_{n}}}$

This may be expressed using matrices:

$\begin{bmatrix}R_{1} \\\vdots \\\vdots \\R_{N}\end{bmatrix} = {\begin{bmatrix}L_{11} & \ldots & \ldots & L_{1N} \\\cdots & \cdots & \cdots & \cdots \\\cdots & \cdots & \cdots & \cdots \\L_{N\; 1} & \ldots & \ldots & L_{NN}\end{bmatrix} \cdot \begin{bmatrix}S_{1} \\\vdots \\\vdots \\S_{N}\end{bmatrix}}$

The vector [S₁ . . . S_(N)] can be solved for using any of the very wellknown methods for solving simultaneous equations.

These operations should be done using images that have already hadbackground cancellation performed. Further integration may have beenalso been performed, or further integration may be performed afterwards.

Also, these operations need to be done for each pixel or group of pixelswithin the chosen integration area. Subsequent processing is the same asa single laser system, except that N sets of data are processed. Thesubscript of S_(n) determines the set to which the particular S valuebelongs.

A typical system may incorporate laser source spot and target spotmonitoring for fault detection and alignment monitoring or feedback.This can still be done even if there is overlap of the spots in theimage by using the computations described above on the relevant pixelsor groups of pixels, provided that the camera is operating in asufficiently linear manner, without excessive saturation.

If saturation makes the separation of the contributions from thedifferent laser spots impossible, then an alternative is to occasionallyswitch only one laser on at a time to confirm the positions of thespots.

In another preferred aspect the present invention provides an apparatusand method of detecting particles comprising emitting a beam ofradiation into a monitored region and detecting a variation in images ofthe region indicating the presence of the particles wherein at least oneof a radiation source and a means for detecting the images is adapted tobe positioned in a controlled manner.

In a preferred embodiment either one of or, both a light source and areceiver are mounted on position control mechanisms to direct theprincipal axis of the receiver and light source and their field of view.The advantage of this is that under either manual or automatic control,the system can be made to more closely examine areas of interest in ascene to better supervise critical areas. This may be implemented as apanning or a tilting mechanism or zoom mechanism or any of a combinationof the three. For example a panning mechanism allows monitoring of awide area of interest which may be beyond the field of view of low-costwide-angle lenses.

FIG. 25 shows an example of a system wherein this pan-tilt-zoom, or PTZis in operation and set up for normal use. The systems comprises a zoomlens(es) 251, a pan-tilt mechanism(s) 252, mounting brackets 253,receiver(s) 254 (preferably in the form of an image capture device suchas a camera), light source(s) 255 (preferably in the form of alaser(s)). Each receiver has a field of view 257 and each source 255produces a beam 254. The system is used to monitor environment 258

FIG. 26 shows the example system of FIG. 26 and how the field of view267 may be adjusted for close examination of a region of interest, whichin this example contains smoke plume 2610.

When the principal axis of either or both of the light source and thereceiver is changed, the system calibration is altered and must beaccounted for in the measurement process for both the physical locationof the region of interest in 3D space as well as the intensity andcharacteristics of the light scatter measurements. This is readilyachieved by either direct calculation or equally by the use of a lookuptable.

The PTZ mechanism offers three degrees of freedom for each of thereceiver and the light source. There are therefore six degrees offreedom in total which can be expressed as a six-dimensional lookuptable. While this is achievable it may be unwieldy in size. For example,allowing for 10 positions in each of the pan, tilt and zoom locations,the possible combinations are 10 to the power 6 or 1 millioncombinations.

Therefore a preferred implementation can use a table of reducedresolution. For example, for five positions in each of pan tilt andzoom, the combination reduces to 5 to the power 6 or 15,625 possiblecombinations. If this is insufficient resolution, it is possible toadditionally apply interpolation to determine values of position thatlie in intermediate points.

Space Calibration

In order to determine the special locations perceived by the system itis necessary to determine the intrinsic and extrinsic parameters of theof the receiver and the light source.

Intrinsic Parameters

In the case where the receiver is a camera using an area array sensor,such as a CCD or CMOS sensor, the important intrinsic parameters are thefocal length, x and y pitch of the sensor elements, the coincidencepoint of the principal axis of the lens and the image array and theradial lens distortion factor. Other parameters such as tilt of theimage sensor plane with respect to the lens principal axis and higherorder effects such as tangential lens distortion may be accounted forbut are generally ignored due to their low significance on measurementresults.

Intrinsic parameters may be determined at manufacture and applied in thefield.

Extrinsic Parameters

Extrinsic parameters must be calibrated in situ as they depend on themode of mounting of the light source and receiver. The parameters thatneed to be measure for full determination of space location are, foreach source and receiver, the effective centre of rotation of the sourceor receiver, X, Y, Z, and the rotation around each of the Cartesianaxes, alpha, beta, gamma.

If these are known along with the intrinsic parameters, it is possiblefor any pixel in the image where the source light beam is visible, todetermine within known limits, the X, Y and Z location of the points inspace being observed.

In another preferred aspect the present invention provides an apparatusand method of detecting particles comprising emitting a beam ofradiation into a monitored region and detecting a variation in images ofthe region indicating the presence of the particles wherein the imagesare detected by image detectors located in at least two positions.

A problem, which may arise in practice, is that the system maymisinterpret images and produce an erroneous particle measurement. Anexample is shown in FIG. 27 in which the monitored environment isindicated by 278. The moving person shown in FIG. 27 may produce imageartifacts that may effect the scatter measurements. Receiver 271 viewsboth the scatter from the beam and the person walking in the background.Although image subtraction will reduce the effects of such interference,the resultant image will appear as shown in FIG. 28. In the image ofFIG. 28, there are intensity changes in the area of the expected beamlocation. This may interfere with the measurements at that point and maylead to false alarms.

A second camera viewing the same scene but from a different vantagepoint as shown in FIG. 29, may be used to verify an alarm and to discardinterference of the type described above. The interference visible inthe second image is clearly not coincident with the expected beamlocation. Since the system does not perceive scatter activity along theline in both images, then it may discard the false information from thefirst camera 271 thus avoiding a false alarm.

A further problem is that it is possible for a bright background tooverwhelm the light scatter from a smoke event causing it to be missed.FIG. 30 illustrates this situation in which environment 308 ismonitored. Prior to any processing that may occur, camera 301 is blindedby the bright light from the window 302 along part of the beam pathrendering it to be ineffective at picking up scatter over that region.The raw image from camera 301 prior to any processing would appear asshown in FIG. 31. In this image, where the beam 311 passes the windowpane 312 the receiver is saturated and therefore unable to detect anyadditional light scatter which might be caused by smoke particlescoincident with the beam at a location where it falls across window pane312 in the image. The second camera viewing the same scene but from adifferent vantage point may be used to cover the area missed by thefirst camera. For example, the second camera image will consist of theimage as shown in FIG. 32. In this image the beam path 321 does notoverlap the window image 322 so the camera is able to pick up smokeparticle events along the length of the beam 321.

As discussed hereinabove, in order to minimise interference effects dueto, for example, local changes in lighting conditions, it is desirableto confine the image processing to the region in an image known to beoccupied by the light source beam and the region nearby. This also hasthe advantage of reducing the computational burden on the processingmeans. It is possible, according a preferred embodiment of theinvention, to calibrate the receiver and light source in such a way thatthe image region where the beam is visible is known. An alternativeapproach is to explicitly determine the position of the beam by knowingtwo points in the beam path. One point can be the light source itselfwhile the other may be a reflective or translucent target or probedisposed in such a manner that it intercepts the path of the beam inspace while remaining in the field of view of the receiver. An exampleof this is shown in FIG. 33 where region 338 is monitored.

The image captured by receiver 343 of FIG. 33 is shown in FIG. 34. Aprobe 346 essentially the equivalent of a scattering feature such as asheet of plastic or glass with suitable scattering characteristics isinterposed in the light beam path 342 in such a way that the beamscatter from the projected light spot 346 is visible to the receiver343. The light source aperture 341 is also visible within the image.Note that the light from light source 341 may be either glare resultingfrom scatter at the exit pupil of the light source or by a specificallyplaced light source, such as an LED. It should also be noted that themeans of suspension of the scattering means (probe) is unimportant aslong as the projected spot 345 remains in the field of view of thecamera 343 at all times. Further it should be noted that the projectedspot 345 may be used to supervise the beam path 342 since the absence ordiminishing of the intensity of the spot 345 may indicate the presenceof an obstruction, which in turn may reduce the detection performance ofthe system.

Where an LED is used as the light source marker, a further feature isthat the LED may be flashed on and off in a manner that allows detectionof it in conditions of high ambient light. For example, subtraction ofan “OFF” image from an “ON” image, in whole or in the part of the imagecontaining the LED, will improve the detection of the LED. Bydetermining the source and destination of the light beam andrespectively the area of interest in the image is easily found by linearinterpolation. In the case where the receiver lens suffers extreme lensdistortion, most commonly radial lens distortion, the interpolation usedmust be of a higher (general second) order rather than being based on astraight line. Radial distortion can be either of barrel distortion ofpincushion distortion as noted hereinabove. In either case, a measure ofthis value as well as other intrinsic parameters may be required inorder to property determine the path of the beam through the image.

The correction applied for radial lens distortion is of the form:r′=r+nr ²

where r′ is the corrected radius, r is the observed radius from theprojection of the principal point in the uncorrected image and n is aconstant found by experiment. In the case of barrel distortion, n is anegative constant. In the case of pincushion distortion, n is a positiveconstant. Such correction methods would be well known to those skilledin the art of image processing and image acquisition.

It may be necessary to monitor the direction of the light source beamrelative to the receiver in order to be able to calculate the level ofsmoke based on received illumination. It is also desirable to monitorthe light beam arrival to ensure that it is not obstructed. A means ofsupervising the light beam, in one embodiment is to observe itsprojection on a surface near the receiver. This was discussedhereinabove and is further elaborated here through the example of analternative embodiment, illustrated in FIG. 36.

Light source 351 projects a beam of light 353 to an area in proximityto, but not directly at, the receiver 352. The projected light spot 354on the wall adjacent to the receiver is not visible to the receiver 352.In the above configuration, therefore, the receiver 352 cannot be usedto verify that the light beam is unobstructed. There are a number ofways in which the arrival of the beam may be supervised.

One embodiment is shown in FIG. 36 where, a rear-view mirror 365 isplaced forward of the receiver 362 such that part of its field of viewis diverted to be rearward-looking. As before, the light beam projectionspot 364 falls to the rear of the receiver 362 but the minor 365reflects its image to the receiver 362 so that it is visible. The imagecaptured by the receiver 362 is shown in FIG. 37. The reflection of theprojected spot 364 is visible in the mirror 365 as is the light source361. In an alternate embodiment, the spot 364 may be supervised using aspecially designed optical element such as a lens capable of observingthe spot image as well as the main forward image. Such a lens is shownFIG. 38. In FIG. 38, the lens housing 381 contains a forward-lookinglens 383 and a rearward-looking lens 384. Light, which enters throughthe forward lens 383, passes through beam splitter 386 and falls uponthe image sensor 387. Light entering through rearward-looking lens 384is reflected by mirror 385 and partially reflected by beam splitter 386and falls upon image sensor 387. The result is a combined image showingboth the spot on the wall to the rear of the receiver and the scene inthe forward direction. The beam splitter 386 may take any of a number ofwell-known forms, such as a prism, but is preferably a section ofparallel sided glass. Such glass may be partially silvered if requiredto better capture light from lens 384 but this is not necessary.

A disadvantage of the above method is that the combination of the twoimages may cause some interference reducing the sensitivity of thereceiver to light scatter in the main direction of view.

An improvement therefore, is to apply a shutter to either or both therearward and forward looking apertures do that they may be observed bythe same receiver in alternation. An example of this is shown in FIG.39. The addition of shutters 388 and 389 allows independent viewing ofthe forward and the rearward scenes. The shutters may be operatedmechanically using motors or other physical actuation means, or may besolid state shutters, having no moving parts, such as a Liquid Crystalshutter or Magneto-Optical shutter.

In an alternative embodiment of this principle, the forward-lookingshutter 388 may be omitted. When it is desired to observe spot 382through rearward looking lens 384, shutter 389 is opened allowing lightfrom the spot to fall on the image sensor. Usually the spot will be farmore intense than any feature in the forward looking scene and is easyto discriminate.

In yet another embodiment, the beam may be supervised using an activeimage capture system. For example, a dedicated camera may be used forthe sole purpose of determining the position and intensity of theprojected spot. This is shown in FIG. 40. Receiver 405 monitors theposition and intensity of the projected spot 404. In one suchembodiment, the rearward-looking receiver 405 may be camera such as aCCD or CMOS array camera or equivalent. In another embodiment of thisprinciple, the receiver may be a Position Sensitive Diode (PSD) wherethe output signal derives from the intensity and position of the spotprojected on its surface. In yet another embodiment of this principle,the receiver 405 may be a single photodiode aligned to observe thereflected spot and to provide a signal based on the spot's intensity.The absence or attenuation of the spot giving rise to an alarm signalthrough the aid of a simple processing means.

In yet another embodiment, the receiver 405 may be an array of two ormore photodiodes, the comparative signals of which may be used toindicate the extent of deviation from of the spot from the desiredlocation.

In any of the above embodiments, the rearward-looking receiver 406 andthe forward-looking receiver 402 may be combined into one physicalstructure for ease of mounting and alignment.

Supervision of Beam by Receiver

In a further embodiment, the same receiver used for detecting scattermay supervise the beam arrival. This is shown in FIG. 41. In thisembodiment of a beam supervisory system, the beam 413 is periodicallysteered to position 415 directly into or near to the lens of thereceiver 412. This may cause a very high signal level, which is used toconfirm the arrival of the light beam. After confirmation the beam issteered back to its normal position 414. The advantage of this approachis that it reduces cost by eliminating the need for a separate beamsupervisory element.

Yet another means of supervising the arrival of the beam is toperiodically direct it to a surface in the field of view of thereceiver. In FIG. 42 two lines 426 indicate the limits of the receiver's422 field of view. In the normal state, the light source 421 directs thebeam 423 to a first target position 424. Periodically, the beam issteered to a second target position 425, which is selected so as to bein the field of view of the receiver 422. The projected spot at 425 isdetected by the receiver, so confirming the arrival of the beam. Thebeam is then returned to its normal position 424.

It would be appreciated by those skilled in the art of particle scattermeasurement, that a beam of light passing through a cloud of particlesis scattered in a manner depending on the light spectrum and the sizedistribution and absorption characteristics of the particles aspreviously discussed hereinabove. The diagram of FIG. 43 shows the imageand beam profile for a beam with no interfering particles present. Inthe diagram, the light spot 431 is present an target 432. A profile ofintensity taken, for example, along line 433 is shown as relativeintensity on graph 434. Where the beam is substantially monochromaticand the particle distribution single-moded where the mode representslarge particles compared with the wavelength of the beam, a pattern offringes is readily observable. In reality, due to inconsistency in theviewing distance and wide distribution of particle sizes, the fringesmerge causing an apparent spreading of the beam. Where the beam spot isobserved on a wall or other target, the effect is to increase theintensity of light in the region surrounding the beam and to reduce theintensity of the spot itself, which is shown in FIG. 44. By combiningthe observed intensity distribution measured above with intensityinformation derived from receivers placed at a number of angles relativeto the beam direction it is possible to form an estimate of the particlesize distribution and also to more closely emulate the reading thatwould be obtained from a standard obscurometer in the same environment.

Suppression Disc

In order to improve the sensitivity to the beam spreading effect, it ispossible to focus the main beam on a light-absorbing structure orsurface or masking structure, so as to accentuate the spreading of thebeam caused by the scatter of large particles. An example of a suitabletarget with this characteristic would be as shown in FIG. 45, where 451is the normal target surface and 452 is a circle of light-absorbingmaterial. Note that equally, 452 may be a cavity structured in such away as to minimise reflection of light back through the aperture. InFIG. 46, the graph 464 represents the intensity profile observed acrossline 463. The effect of beam spread is more readily detectable by thereceiver due to the suppression 465 of the very bright central spotallowing the detection of the dimmer tails 466.

Test Illuminator to Check Receiver

It may be necessary to ensure that the receiver is operating correctly.Where the receiver is an area array detector, such as a COD or CMOScamera, defective picture elements (pixels) or excessive dust particlessettling on the image array surface may cause the system to miss lightscatter events.

In one embodiment, a means of checking the operation of each element isto provide an illumination source to flood the array with light. Eachelement may be checked against an acceptable standard and a pass/failassessment made. An improvement to this test is to store a referenceimage from the receiver with the illuminator active at an early stage ofmanufacture or installation and use this stored frame for comparisonwith subsequent illumination test frames eliminating the need tocompensate specifically for minor pixel-to-pixel variations or staticspatial variations in illumination during the test.

One means of checking the operation of the array is to provide anexternal light source, which may be periodically disposed in front ofthe receiver to cast an even glow. In FIG. 47 the illuminator means 474is temporarily disposed ahead of lens housing 471. Light from anillumination source 476 passes through optional screen 475 which servesto scatter the light from said illumination source which subsequentlypasses through lens system 472 and on to image sensor 473 where saidimage sensor is capable of spatially resolving intensity variations overits surface, as for example a CCD or CMOS array. The illuminator means474 may be implemented in a number of ways using light sources such aselectroluminescent panels, LEDs or where there is sufficientenvironmental illumination, the said means may comprise a simple groundglass screen or equivalent to scatter the illumination already presentin the environment surrounding the receiver.

Yet another means of implementing the test illuminator is shown in FIG.48. In FIG. 48, an illumination source 486 is placed in close proximityto the receiver detector array, in the space between the lens system 482and the image receiver 483. This illuminator may be activatedperiodically and the functioning of the image array checked.

Backscatter to Detect Thick Plumes of Smoke

In the event of a sudden thick plume of smoke as may occur in whenhighly flammable material is ignited, it is possible that the light beamwill be so greatly attenuated that the forward scatter will beundetectable. Under these conditions it is possible, according to afurther embodiment of the invention, to use the light scattered backtoward the source to indicate the location and quantity of smoke in theair as discussed hereinbefore.

An example of this configuration is shown in FIG. 49. Referring to FIG.49, a light source 491 projects a beam 492 through space to point 493located near receiver camera 494. Smoke plume 495 has an opticalobscuration so that no significant amount of light from the beam isdetectable by the receiver camera 494. However, an additional receivercamera 496 is placed adjacent to light source 491 so as to receive lightemanating as backscatter from the dense plume. This allows detection ofthe plume as smoke and subsequent raising of an alarm.

An alternative implementation of the same method is shown in FIG. 50where, the light beam 502 from source 501 is totally obscured by smokeplume 506. Secondary light source 503 next to receiver 504 is made toproject a beam 505, which enters the plume, Backscatter from beam 505 isdetected by receiver 504, which is made to raise an alarm.

Due to the low levels of scattered light relative to the background inan image, it is necessary to apply algorithms to reduce the effects ofimage noise thus improving the detection capability of the system. Thisprocess may be explained with reference to the FIG. 51. Where no scalingis employed, the first image 511 is captured with the light source off.In the second image 512, the image is captured with the light source 514on and under identical ambient lighting conditions. The difference image513 formed by subtracting 511 from 512 shows no background artifacts butallows the light source scatter to be easily detected. The receivingsystem's sensitivity may ordinarily be adjusted to ensure that thecaptured images are within its dynamic range. Where interference occurs,the overall background intensity may differ between the laser-on andlaser-off images. When the image subtraction is performed, therefore,the background does not cancel out completely and so backgroundartifacts remain in the difference image. In the FIG. 52, image 521 withthe light source off has a higher overall intensity due to, for example,fluorescent light flicker. Image 521 with the light source off issubtracted from image 522 with the light source 524 on, revealingresultant image 623. In the resultant image 523 features from thebackground are not cancelled by the subtraction process. This may leadto erroneous detection events or alternatively may reduce the ability ofthe system to discern smoke events due to the need to set higherthresholds for detection.

A means of overcoming this is to apply a correction based on theintensity of the images which are known to be equivalent from one imageto the next. Comparing the background image (light source is off) withthe active image (light source is on) it is clear that there are areasin both images which do not change due to the illumination of the lightbeam. Therefore, any variation in these areas must be due tointerference effects such as fluorescent light flicker. In FIG. 53, theshaded region 531 represents an area known to exclude the area of thebeam path 532. Region 531 is called the Background Integration Area, andregion 532 is called the Light Beam Integration Area. By evaluating theillumination in 531 in an image it is possible to adjust the whole imageso that its intensity is increased or reduced as required to make thereference region 531 have a desired illumination. This may be regardedas a form of automatic gain control. Therefore, when such processedimages are used for image subtraction, the resultant image more readilyreveals the scatter from the light beam in the area 533.

In an alternate implementation, the images may be adjusted at the timeof subtraction without first having to modify the images. This may leadto some economy in arithmetic processing. An example of this is asfollows.

Let there be two images, I1 and I2 where I1 is the image with the lightbeam off and I2 is the image with the light beam on. Let the referenceregion of image 531 be R1 and the reference image of I2 be R2. Further,let the average intensity of all of the pixels in R1 be V1 and let theaverage intensity of all of the pixels in R2 be V2. Then, the differenceimage Idiff may be formed by the calculation

${I_{diff}( {x,y} )} = {{I_{2}( {x,y} )} - \frac{V_{2}{I_{1}( {x,y} )}}{V_{1}}}$

for each pixel (x,y)

This step corrects for overall changes in illumination so that thedominant feature in the difference image is the scatter from the lightsource beam.

A further enhancement of this method is to confine the arithmeticprocessing to the Light Beam Integration Area. This reduces thecomputational load permitting a more economical implementation.

A better measure of variation may be obtained by using reference regionson either side of the light beam position. In FIG. 54, the regions 541and 542 on either side of the beam region 543 are used to track relativechanges between images. Since the detection algorithm preferablycompares an image with the beam turned off with an image where the beamis on, this has particular application where there is interference dueto external light sources, such interference being unsynchronised withrespect to the image capture times. Examples of such interference arefluorescent lights and neon signs. Where these interference sourcesexist, it is possible to scale images taken with the light source, onand with the light source off, so that subtraction of images will morecompletely remove image background artifacts.

Where the path of the beam 543 in the detecting image is known, regions541 and 542 on either side of it may be used as a measure of overallillumination in the image. The correction formula is the same as thecorrection formula given above.

A further enhancement of this method allows for corrections where theinterference is not even over the image area. For example, if there isan interfering light source disposed so as to predominantly illuminateone region of the area being monitored, an overall or global adjustmentmay not be possible. Therefore, a localised or region-by-regioncorrection is better applied.

This will be explained with reference to FIG. 55. The images are dividedup in to sections above 551 and below 552 the position of the beam 553.Corrections of the type described above are now applied on aregion-by-region basis where each region consists of a section of type551 and a section of type 552 below it. Thus each region [4] to [12]comprises a triplet of a section 551, a section 562 and the regionbetween 551 and 552 where the beam path exists.

The correction formula is then calculated and applied on aregion-by-region basis, being applied only to the pixels in the regionapplicable.

Where ratiometric correction and subsequent background cancellation areapplied, there are four elements to the calculation each having anunwanted noise component as well at the wanted signal. The elements arethe Light Beam Integration Area with the light source on, the Light BeamIntegration Area with the light source off, the Background IntegrationArea with the light source on and finally the Background IntegrationArea with the light source off.

The noise in the system mainly arises from receiver image noise. Thismay be reduced by capturing and integrating a number of images, byincreasing the size of the integration regions or by increasing the dutycycle of the light source on time. These measures may be usedindividually or in combination to improve the signal with respect to thereceiver noise.

In order to achieve optimum noise reduction it is important that theregions selected for calculation are not prone to excessiveinterference.

Excessive interference could arise from objects in the field of viewsuch as televisions, computer monitors, animated signs and so on. Otherobjects may also present interference, such as moving machinery, anexternal window to passing traffic or a walkway in the field of viewwith regular pedestrian traffic.

During installation or commissioning, it is possible to nominate,manually, areas to exclude from processing. Thereafter the system mayignore data from the excluded regions.

In a preferred implementation, the selection of the excluded regionswould be automated removing the need for manual setup of this aspectduring installation or commissioning. Each picture element may becharacterised by a parameter, which measures its level change over time.Such a measure may be obtained by calculating the standard deviation ofthe pixel level over a selected period. Where such measure for a givenpixel is significantly in excess of the majority of pixels, that pixelwould be marked as unsuitable for use in region calculations.

It is desirable to monitor the position of the light beam source in thereceivers field of view in order to be able to be able to predict thebeam path in the received image. This may be done as describedhereinabove with reference to FIG. 33, where knowing the position of thesource beam and at least one other point in the beam path, and area ofinterest can be identified corresponding to the path of the beam throughthe image.

A significant problem that arises when viewing the laser source is thatthe receiver can be overloaded so that the image captured is saturatedin the region of the source. The result of this is that the region soeffected is not sensitive to scatter information and is therefore unableto detect smoke. In FIG. 56, the light source 561, generating beam 562overloads a large portion of the received image and this effectivelydisables smoke detection in that region.

The problem can be alleviated by masking the light source in such a wayas to shield the receiver from light directly scattered from the sourceaperture. One method for masking the light source is to apply a seriesof baffles in line with the light source aperture. With reference toFIG. 57, a system of at least one baffle plate 572 is placed at theaperture of a light beam source 571. The main light beam 573 passesunhindered. Off-axis light scatter 574 is absorbed by the baffle system572 and is therefore not visible to receiver whose viewing direction isshown by 575.

The use of such a system of baffles greatly reduces or altogethereliminates the image of the light source captured by the receiver.

Devices other than a baffle system can be used to achieve an equivalentresult. For example a simple opaque or semi-opaque plate can be placedso that it shades the direct view of the light source aperture by thereceiver, but does not interfere with the passage of the main beam. Thisis shown in FIG. 58, where plate 582 intercepts and absorbs and sidescatter 584 that would be received along receiver viewing angle 585. Theuse of a semi-opaque plate has the advantage that the location of thelight source can sill be identified in the receiver's image due to thelight passing through the plate from the light source to the receiver.

While this invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification(s). This application is intended to cover any variationsuses or adaptations of the invention following in general, theprinciples of the invention and comprising such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth.

As the present invention may be embodied in several forms withoutdeparting from the spirit of the essential characteristics of theinvention, it should be understood that the above described embodimentsare not to limit the present invention unless otherwise specified, butrather should be construed broadly within the spirit and scope of theinvention as defined in the appended claims. Various modifications andequivalent arrangements are intended to be included within the spiritand scope of the invention as described hereinabove. Therefore, thespecific embodiments are to be understood to be illustrative of the manyways in which the principles of the present invention may be practiced.In the description hereinabove, means-plus-function clauses are intendedto cover structures as performing the defined function and not onlystructural equivalents, but also equivalent structures. For example,although a nail and a screw may not be structural equivalents in that anail employs a cylindrical surface to secure wooden parts together,whereas a screw employs a helical surface to secure wooden partstogether, in the environment of fastening wooden parts, a nail and ascrew are equivalent structures.

“Comprises/comprising” when used in this specification is taken tospecify the presence of stated features, integers, steps or componentsbut does not preclude the presence or addition of one or more otherfeatures, integers, steps, components or groups thereof.

The invention claimed is:
 1. A method of detecting particles comprising:emitting a beam of radiation into a monitored region; and detecting avariation in images of the region with image capturing means such thatthe variation in images indicates the presence of the particles, whereinthe steps of emitting and detecting comprise determining an ON period ofthe beam of radiation and an exposure period of the image capturingmeans on the basis of a power level of the emitted beam.
 2. A method asclaimed in claim 1 wherein the method further includes, modulating thebeam of radiation such that the ON period of the beam is synchronizedwith the exposure period of the image capturing means.
 3. A method asclaimed in claim 2 wherein the ON period of the beam is one of greaterthan, less than or equal to the exposure period.
 4. A method as claimedin claim 1 wherein the beam is pulsed.
 5. A method as claimed in claim 1further comprising one or more of the following steps: providing an ONperiod of the beam in the range of about a few seconds to 0.1 msecs;providing a corresponding beam power level in the range of 1.18 mW to500 mW.
 6. A method as claimed in claim 1 wherein the image capturingmeans operates with a frame rate in the range between 25 and 100 framesper second and the beam is pulsed at a frequency in a range between 12.5Hz and 50 Hz.
 7. A method as claimed in claim 1 wherein the particlescomprise smoke particles.
 8. A method as claimed in claim 1 wherein theimage capturing means comprises one or more image capture devices.
 9. Amethod as claimed in claim 1 wherein the image capturing means comprisesat least two image capture devices viewing the beam of radiation fromopposing positions.
 10. A method as claimed in claim 1 furthercomprising the steps of: providing a first beam emitter paired with afirst image capture device; providing a second beam emitter paired witha second image capture device; monitoring the emitted beam of the firstemitter with the second image capture device; and monitoring the emittedbeam of the second emitter with the first image capture device.
 11. Amethod as claimed in claim 10 wherein one or more of the first andsecond paired emitters and image capture devices are provided in one andthe same mounting arrangement.
 12. A method as claimed in claim 11further comprising the step of: pre-aligning one or more of the firstand second paired emitters and image capture devices.
 13. A method asclaimed in claim 12 wherein the step of pre-aligning comprises: settinga beam direction of respective emitters such that the beam emerges at apredetermined angle with respect to the centre of view of thecorresponding image capture device.
 14. A method as claimed in claim 13wherein the predetermined angle is θ, the distance between respectiveemitter/image capture device pairs is D meters such that a distancebetween a target spot of the beam of the first emitter and the secondimage capture device is given by: S=D tan θ.
 15. A method as claimed inclaim 2 which further includes synchronising the emission of the beam ofradiation and the image capturing means comprises: emitting the beam ofradiation at a pre-determined modulation rate; identifying a radiationsource, being the source of the beam of radiation, in one or more videoimages captured by the image capturing means; and continually modifyingthe image capturing means frame rate to remain in synchronisation.
 16. Amethod as claimed in claim 15 wherein the radiation source has asecondary light source mounted to it, said secondary light sourcecomprising a wide angle of emission.
 17. A method as claimed in claim 16wherein the step of identifying the radiation source in one or morevideo images captured by the image capturing means comprises: flashingthe secondary light source in a predetermined timing relationship withthe modulation of the radiation source to facilitate locating theradiation source in the image capturing means image.
 18. A method asclaimed in claim 17 wherein the secondary light source flashes in one ormore of: a periodic pattern; a pseudo-random sequence.
 19. A method asclaimed in claim 17 further comprising the steps of: providing the imagecapturing means with a frame rate that approximates an oscillation rateof an oscillator of the secondary light source; identifying the flashingsecondary source; varying a phase of the image capturing means framerate to maintain synchronization between the exposure period of theimage capturing means and the flashing of the secondary light source.20. A method as claimed In any one of claims 19 wherein a frequency ofthe secondary light source oscillator is altered to be one of: the sameas; a multiple of; a sub multiple of an AC mains electricity supplyfrequency.
 21. A method as claimed in claim 1 further comprising thesteps of: determining a path loss measurement by recording the intensityof a target incident beam spot of the emitted beam within the capturedimage and; comparing the recorded intensity with a recorded intensity ata previous time.
 22. A system for detecting particles in monitoredregion, including one or more light sources configured to at leastperiodically emit a beam of radiation into the monitored region, one ormore image capture devices for collecting an image of the region beingmonitored during an exposure period, an image processor for determininga variation in the images to detect the presence of particles at aplurality of locations within the region wherein at least one of theduration and timing of the emission of the beam into the monitoredregion and exposure period are determined on the basis of a power levelof the emitted beam.
 23. A system for detecting particles as claimed inclaim 22 wherein the emission of the beam is synchronised with theexposure period of the image capturing means.
 24. A system for detectingparticles as claimed in claim 22 wherein the beam is pulsed.
 25. Asystem for detecting particles as claimed in claim 22 wherein theduration of emission of the beam into the region is in the range of afew seconds to 0.1 msecs; and the corresponding beam power level is inthe range of 1.18 mW to 500 mW.
 26. A system for detecting particles asclaimed in claim 22 wherein the image capturing means operates with aframe rate in the range of 25 to 100 frames per second and theperiodicity of emission of the beam is in a range of 12.5 Hz to 50 Hz.27. A method as claimed in claim 1 wherein the ON period of the beam ofradiation and an exposure period of the image capturing means isdetermined in accordance with an indirectly proportional relationshipwith the power level of the beam.